cleanup watchrule delete logic/tests

cleanup watchrule delete logic/tests

Rework watchrule delete interface so we delete by ID, rather than
by name (where we only delete the first rule in the event of multiple
watcherule rows with the same name, which is allowed although not likely
in normal heat operation - it can cause weird behavior in the tests)

Encapsulating the delete in a WatchRule.destroy() is cleaner and more
consistent with the rest of the codebase, and reworking the tests to
ensure proper cleanup reduces the chances of odd errors and cascading
failures between tests on failure.

fixes bug #1197718

Change-Id: I94e9eb610519bb7321a2be7718312fd50f308a8b