Instance + Flavor Plugin SPI Changes

[chungers]

This PR introduces the following incompatible API changes to the Instance and Flavor Plugins:

New Label() Method Requirement for Instance Plugin

Addition of a new Label() method requirement on the Instance Plugin SPI. This adds a new requirement for Instance plugins to implement method for labeling / tagging a resource:

type Plugin interface {
    // existing methods
    Label(instance instance.ID, labels map[string]string) error
}

• This new method is necessary to support features for InfraKit to "import" (or assimilate) resources that have been created outside of InfraKit.
• It is also needed to ensure bootstrapping node is labeled properly to address some issues with rolling updates.

Use *types.Any instead of *json.RawMessage

• Replace *json.RawMessage in the methods of Flavor and Instance plugin methods with *types.Any.
• Makes consistent use of pointers in method parameters throughout and standardizes on a single abstraction of types.Any to represent user-defined, opaque structs.
• Removes lots of boilerplate code for JSON unmarshalling.
• This will make it easier to support other formats (e.g. YAML or GRPC) since we only need to change the encoding and decoding of data in a single location.

INSTANCE PLUGIN DEVELOPERS
Instead of the following boilerplate code

func (p *myPlugin) Validate(spec *json.RawMessage) error {
    mySpec := someStruct{}
    err := json.Unmarshal([]byte(*spec), &mySpec)
    if err != nil {
       return err
    }
    // ....  Do validate
}    

Use this instead:

func (p *myPlugin) Validate(spec *types.Any) error {
    mySpec := someStruct{}
    err := spec.Decode(&mySpec)
    if err != nil {
       return err
    }
    // ....  Do validate
}    

[GordonTheTurtle]

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

$ git clone -b "spi-tagging" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353855840
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-io]

Codecov Report

Merging #371 into master will increase coverage by -2.77%.

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
- Coverage   66.34%   63.57%   -2.77%     
==========================================
  Files          47       52       +5     
  Lines        2371     2817     +446     
==========================================
+ Hits         1573     1791     +218     
- Misses        629      805     +176     
- Partials      169      221      +52

Impacted Files	Coverage Δ
pkg/plugin/group/types/types.go	32.35% <ø> (+6.77%)	✅
pkg/rpc/group/service.go	66.66% <ø> (ø)	✅
pkg/rpc/server/reflector.go	68.05% <ø> (ø)	✅
pkg/types/any.go	70.45% <ø> (-7.05%)	❌
examples/instance/terraform/softlayer.go	100% <100%> (ø)
pkg/plugin/group/scaled.go	70% <100%> (-1.7%)	❌
pkg/rpc/instance/client.go	94.28% <100%> (+0.95%)	✅
pkg/rpc/flavor/client.go	93.93% <100%> (ø)	✅
pkg/plugin/group/group.go	68.29% <25%> (ø)	✅
examples/flavor/combo/flavor.go	51.31% <27.27%> (ø)	✅
... and 15 more

---

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