Add skopeo login&logout

Implements skopeo login&logout commands.

Signed-off-by: Qi Wang <qiwan@redhat.com>

docs/skopeo-login.1.md

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+% skopeo-login(1)
+
+## NAME
+skopeo\-login - Login to a container registry
+
+## SYNOPSIS
+**skoepo login** [*options*] *registry*
+
+## DESCRIPTION
+**skopeo login** logs into a specified registry server with the correct username
+and password. **skopeo login** reads in the username and password from STDIN.
+The username and password can also be set using the **username** and **password** flags.
+The path of the authentication file can be specified by the user by setting the **authfile**
+flag. The default path used is **${XDG\_RUNTIME\_DIR}/containers/auth.json**.
+
+## OPTIONS
+
+**--password**, **-p**=*password*
+
+Password for registry
+
+**--password-stdin**
+
+Take the password from stdin
+
+**--username**, **-u**=*username*
+
+Username for registry
+
+**--authfile**=*path*
+
+Path of the authentication file. Default is ${XDG\_RUNTIME\_DIR}/containers/auth.json
+
+Note: You can also override the default path of the authentication file by setting the REGISTRY\_AUTH\_FILE
+environment variable. `export REGISTRY_AUTH_FILE=path`
+
+**--get-login**
+
+Return the logged-in user for the registry. Return error if no login is found.
+
+**--cert-dir**=*path*
+
+Use certificates at *path* (\*.crt, \*.cert, \*.key) to connect to the registry.
+Default certificates directory is _/etc/containers/certs.d_.
+
+**--tls-verify**=*true|false*
+
+Require HTTPS and verify certificates when contacting registries (default: true). If explicitly set to true,
+then TLS verification will be used. If set to false, then TLS verification will not be used. If not specified,
+TLS verification will be used unless the target registry is listed as an insecure registry in registries.conf.
+
+**--help**, **-h**
+
+Print usage statement
+
+## EXAMPLES
+
+```
+$ skopeo login docker.io
+Username: testuser
+Password:
+Login Succeeded!
+```
+
+```
+$ skopeo login -u testuser -p testpassword localhost:5000
+Login Succeeded!
+```
+
+```
+$ skopeo login --authfile authdir/myauths.json docker.io
+Username: testuser
+Password:
+Login Succeeded!
+```
+
+```
+$ skopeo login --tls-verify=false -u test -p test localhost:5000
+Login Succeeded!
+```
+
+```
+$ skopeo login --cert-dir /etc/containers/certs.d/ -u foo -p bar localhost:5000
+Login Succeeded!
+```
+
+```
+$ skopeo login -u testuser  --password-stdin < testpassword.txt docker.io
+Login Succeeded!
+```
+
+```
+$ echo $testpassword | skopeo login -u testuser --password-stdin docker.io
+Login Succeeded!
+```
+
+## SEE ALSO
+skopeo(1), skopeo-logout(1)
+
+## HISTORY
+May 2020, Originally compiled by Qi Wang <qiwan@redhat.com>

docs/skopeo-logout.1.md

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+% skopeo-logout(1)
+
+## NAME
+skopeo\-logout - Logout of a container registry
+
+## SYNOPSIS
+**skopeo logout** [*options*] *registry*
+
+## DESCRIPTION
+**skopeo logout** logs out of a specified registry server by deleting the cached credentials
+stored in the **auth.json** file. The path of the authentication file can be overridden by the user by setting the **authfile** flag.
+The default path used is **${XDG\_RUNTIME\_DIR}/containers/auth.json**.
+All the cached credentials can be removed by setting the **all** flag.
+
+## OPTIONS
+
+**--authfile**=*path*
+
+Path of the authentication file. Default is ${XDG\_RUNTIME\_DIR}/containers/auth.json
+
+Note: You can also override the default path of the authentication file by setting the REGISTRY\_AUTH\_FILE
+environment variable. `export REGISTRY_AUTH_FILE=path`
+
+**--all**, **-a**
+
+Remove the cached credentials for all registries in the auth file
+
+**--help**, **-h**
+
+Print usage statement
+
+## EXAMPLES
+
+```
+$ skopeo logout docker.io
+Remove login credentials for docker.io
+```
+
+```
+$ skopeo logout --authfile authdir/myauths.json docker.io
+Remove login credentials for docker.io
+```
+
+```
+$ skopeo logout --all
+Remove login credentials for all registries
+```
+
+## SEE ALSO
+skopeo(1), skopeo-login(1)
+
+## HISTORY
+May 2020, Originally compiled by Qi Wang <qiwan@redhat.com>