Migrate GCP plugin implementation to main repo

[chungers]

This PR moves the code and history of the GCP (Google) instance plugin (etc.) into the main infrakit repo. With this, we are also starting a pkg/provider as the way to organize different infrastructure providers. In the future, we will migrate the provider-specific code in pkg/plugin/instance to follow this convention, because this new convention will scale better as each provider will contain implementations of multiple infrakit SPI, not just the interface SPI alone.

• Reorganizes the docker/infrakit.GCP repo and moves the files to pkg/provider/google.
• Contribution history is preserved
• Updated MAINTAINERS to merge entries from source repo

@dgageot @FrenchBen

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "migration-gcp" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353665216
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov]

Codecov Report

Merging #585 into master will decrease coverage by 1.9%.
The diff coverage is 29.33%.

@@            Coverage Diff             @@
##           master     #585      +/-   ##
==========================================
- Coverage   48.86%   46.95%   -1.91%     
==========================================
  Files          78       85       +7     
  Lines        5010     5552     +542     
==========================================
+ Hits         2448     2607     +159     
- Misses       2291     2667     +376     
- Partials      271      278       +7

Impacted Files	Coverage Δ
pkg/provider/google/plugin/gcloud/api.go	0% <0%> (ø)
pkg/provider/google/plugin/gcloud/apiMetadata.go	0% <0%> (ø)
pkg/provider/google/plugin/instance/util/name.go	100% <100%> (ø)
pkg/provider/google/plugin/instance/types/types.go	51.35% <51.35%> (ø)
pkg/provider/google/plugin/instance/plugin.go	72.54% <72.54%> (ø)
pkg/provider/google/plugin/metadata/plugin.go	75% <75%> (ø)
pkg/provider/google/plugin/gcloud/tags.go	82.14% <82.14%> (ø)
... and 4 more

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[FrenchBen]

Overall LGTM but cannot be merged until the Makefile is cleaned up

[FrenchBen]

Invalid git merge :P
Needs to be cleaned out first.