Submitted is _not_ necessarily merged in Gerrit

* zuul/trigger/gerrit.py(Gerrit._isMerged): Only consider MERGED
status to indicate a change is successfully merged. SUBMITTED can
exist on changes which Gerrit was unable to merge due to nonexistent
parent/dependent commits, and possibly for other reasons too.

Change-Id: I673be5010031f8bc26ab31db9afcc7c28854db85
Merge "Submitted is _not_ necessarily merged in Gerrit"