Dynamic and context-driven Infrakit CLI

[chungers]

This PR makes a significant change in the CLI and will be part of the v0.5 release:

• The instance, group, flavor commands have been made context-sensitive. Specifically, these are no longer subcommands on their own anymore. Instead, the CLI will look at all the plugins known and their interface types and generate appropriate commands (e.g. provision for an Instance plugin, a prepare for a flavor plugin). The name of the plugin becomes the subject / command, and the methods/actions (e.g. prepare, validate, etc.) become verbs and are subcommands.
• This makes the CLI context-sensitive and reduces typing and error: all the commands are in context of what is actually possible: there's no point having a instance --name foo provision if the plugin foo isn't running. Instead, the command becomes foo provision and exists only when a plugin named foo of type Instance which supports provision exists.
• This architecture makes the CLI very modular and dynamic. The commands are now in version specific packages (currently v1). This will allow evolution of the system into v2, v3,... without impacting the functionality of older versions.
• This new system also introduced a refactor of the common services for CLI -- template processing, output JSON/YAML transcoding, etc. This makes the flags and behaviors across different command modules consistent.

@YujiOshima @ndegory

[codecov]

Codecov Report

Merging #544 into master will decrease coverage by 0.04%.
The diff coverage is 20%.

@@            Coverage Diff             @@
##           master     #544      +/-   ##
==========================================
- Coverage   56.66%   56.61%   -0.05%     
==========================================
  Files          57       57              
  Lines        3937     3944       +7     
==========================================
+ Hits         2231     2233       +2     
- Misses       1420     1425       +5     
  Partials      286      286

Impacted Files	Coverage Δ
pkg/rpc/mux/reverse_proxy.go	38.21% <0%> (-0.25%)	⬇️
pkg/template/template.go	71.11% <0%> (-1.62%)	⬇️
pkg/cli/remote/remote.go	61.9% <40%> (+0.92%)	⬆️

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