enable tests.storage.test_impl_mongodb
enable tests.storage.test_impl_mongodb

Currently, test cases in tests/storage/test_impl_mongodb.py are all
ignored, since they inherit from tests.db:TestBase, which defines
db_url only for sqlite.

This patch adds mixed scenario tests for the ignored test cases to
enable them on mongodb back end.

NOTE: also fix a test case for mongodb ttl

Change-Id: Ibb74f8455293fffa97a234264e12446e3382e57c
Closes-Bug: #1413066

eayunstack/ceilometer,cernops/ceilometer,fabian4/ceilometer,ityaptin/ceilometer,openstack/aodh,openstack/aodh,mathslinux/ceilometer,pczerkas/aodh,eayunstack/ceilometer,maestro-hybrid-cloud/ceilometer,cernops/ceilometer,idegtiarov/ceilometer,r-mibu/ceilometer,mathslinux/ceilometer,pczerkas/aodh,isyippee/ceilometer,Juniper/ceilometer,pkilambi/ceilometer,openstack/ceilometer,chungg/aodh,pkilambi/ceilometer,isyippee/ceilometer,chungg/aodh,redhat-openstack/ceilometer,maestro-hybrid-cloud/ceilometer,redhat-openstack/ceilometer,openstack/ceilometer,ityaptin/ceilometer,idegtiarov/ceilometer,fabian4/ceilometer,sileht/aodh,sileht/aodh,r-mibu/ceilometer,Juniper/ceilometer