OpenStack Push support

Signed-off-by: Dan Finneran <daniel.finneran@gmail.com>

docs/platform-openstack.md

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+# LinuxKit with OpenStack
+
+LinuxKit interacts with OpenStack through its native APIs and requires access
+to both an OpenStack Keystone server for authentication and a OpenStack Glance
+server in order to host the LinuxKit images.
+
+Supported (Tested) Versions:
+
+- OpenStack Ocata Release
+- Keystone v3 API 
+- Glance v2 API
+
+##Push
+
+### Image types supported:
+- **ami** (Amazon Machine image)
+- **vhd** (Hyper-V)
+- **vhdx** (Hyper-V)
+- **vmdk** (VMware Disk)
+- **raw** (Raw disk image)
+- **qcow2** (Qemu disk image)
+- **iso** (ISO9660 compatible CD-ROM image)
+
+A compatible/supported image needs to have the correct extension (must match
+one from above) in order to be supported by the `linuxkit push openstack` 
+command. The `openstack` backend will use the filename extension to determine
+the image type, and use the filename as a label for the new image. 
+
+The `openstack` backend also supports OpenStack projects to provide
+multi-tenancy support when uploading images. 
+
+### Usage
+
+The `openstack` backend uses the password authentication method in order to
+retrieve a token that can be used to interact with the various components of
+OpenStack. The URLs for the Keystone/Glance server components need to have 
+the ports detailed as below.
+
+```
+./linuxkit push openstack \
+-keystoneAddr=http://keystone.com:5000 \
+-username=admin \
+-password=XXXXXXXXXXX \
+-project=linuxkit \
+-glanceAddr=http://glance.com:9292 \
+./linuxkit.iso 
+```
+
+### Execution Flow
+1. Log in to OpenStack (Keystone)
+2. Retrieve the OpenStack Key from the response header
+3. Create a "queued" image on the glance server and return the new image ID
+4. Use the new image ID and upload the LinuxKit image under this new ID