Alarm rule evaluators are loadable with stevedore, but not the API side,
since wsme 0.6 it's now possible to do it.
This change adds that to allow to create a gnocchi alarm rule api and
evaluator in the gnocchi code tree.
This also move out the V2Controller API from __init__ to avoid
recursive import with stevedore.
Change-Id: I468f9f82b41eb7f4896dd93a8c62ce63b49535bb
2e75325060