Change some assertTrue to assertIsNotNone
Change some assertTrue to assertIsNotNone

Change checks for a valid object from assertTrue to assertIsNotNone
which seems closer to the intent.

Also rename a local variable from "next" (name clash with the
built in function) to "next_time".

Change-Id: Ie7a770c7ac41e7c38e7d50728a7c194e5afec0cd

gilbertpilz/solum,ed-/solum,openstack/solum,gilbertpilz/solum,gilbertpilz/solum,julienvey/solum,stackforge/solum,ed-/solum,devdattakulkarni/test-solum,gilbertpilz/solum,stackforge/solum,ed-/solum,ed-/solum,devdattakulkarni/test-solum,julienvey/solum,openstack/solum