Allow strings to be unmarshalled for CNI_ARGS
CNI_ARGS uses types.LoadArgs to populate a struct.
The fields in the struct must meet the TextUnmarshaler interface.
This code adds a UnmarshallableString type to assist with this.
This commit adds a struct type CommonArgs that is to be embedded in
every plugin's argument struct. It contains a field named
"IgnoreUnknown" which will be parsed as a boolean and can be provided to
ignore unknown arguments passed to the plugin.
This takes some of the machinery from CNI and from the rkt networking
code, and turns it into a library that can be linked into go apps.
Included is an example command-line application that uses the library,
called `cnitool`.
Other headline changes:
* Plugin exec'ing is factored out
The motivation here is to factor out the protocol for invoking
plugins. To that end, a generalisation of the code from api.go and
pkg/plugin/ipam.go goes into pkg/invoke/exec.go.
* Move argument-handling and conf-loading into public API
The fact that the arguments get turned into an environment for the
plugin is incidental to the API; so, provide a way of supplying them
as a struct or saying "just use the same arguments as I got" (the
latter is for IPAM plugins).
