Fix #1011: Group dynamic metrics in cache under appropriate keys

[geauxvirtual]

Fixes #1011

Summary of changes:

• Adds control/strategy/cache_small_test.go file
• Adds TestUpdateCache method to cache_small_test.go to test adding static and dynamic metric entries to the cache
• Reimplement updateCache method to properly store dynamic metrics in cache under the metrics dynamic namespace instead of the namespace returned from the plugin.

Testing done:

• Wrote new tests to verify functionality of code changes.

@intelsdi-x/snap-maintainers

[pittma]

For each dynamic metric, we are doing n2 writes where n is the number of metrics which are a result of the query.

This loop should be moved outside of the outer mts loop, and done iff len(dc) > 0.

[pittma]

LGTM

Landing.