Swarm flavor group and instance plugins

[kaufers]

Swarm instance plugin: Implements DescribeInstances and Destroy only

• DescribeInstances returns all swarm nodes (no filtering) with the swarm node ID as the instance ID, the "link-label" as the LogicalID, all engine labels as the Tags (plus a name tag added with the node hostname), and the entire node struct as the Properties.
• Destroy still needs to be implemented.

Swarm group plugin: Implements DescribeGroup and Size only

• DescribeGroup retrieves all swarm nodes from the instance plugin and excludes any nodes without a LogicalID (those that do no have a "link-label") and and that are not in a ready state.
• Size returns the number of nodes with the same filtering as DescribeGroup.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@3484afd). Click here to learn what that means. The diff coverage is n/a.

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[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 "swarm-group-inst-plugins" git@github.com:kaufers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353359616
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.