mongo: remove _id in inserted alarm changes
mongo: remove _id in inserted alarm changes

This change ensures that mongodb driver doesn't modify the alarm changes
dictionary when we insert a new alarm changes. To avoid the insertion
of the internal '_id' of mongodb.

Change-Id: I9bcd643c9dc55449141cee94ef6826784e81e4e5

mathslinux/ceilometer,redhat-openstack/ceilometer,froyobin/ceilometer,openstack/ceilometer,mathslinux/ceilometer,pkilambi/ceilometer,m1093782566/openstack_org_ceilometer,r-mibu/ceilometer,pkilambi/ceilometer,eayunstack/ceilometer,idegtiarov/ceilometer,r-mibu/ceilometer,fabian4/ceilometer,openstack/aodh,sileht/aodh,openstack/ceilometer,isyippee/ceilometer,eayunstack/ceilometer,openstack/aodh,cernops/ceilometer,pczerkas/aodh,cernops/ceilometer,Juniper/ceilometer,luogangyi/Ceilometer-oVirt,froyobin/ceilometer,m1093782566/openstack_org_ceilometer,chungg/aodh,luogangyi/Ceilometer-oVirt,maestro-hybrid-cloud/ceilometer,Juniper/ceilometer,pczerkas/aodh,MisterPup/Ceilometer-Juno-Extension,MisterPup/Ceilometer-Juno-Extension,fabian4/ceilometer,isyippee/ceilometer,maestro-hybrid-cloud/ceilometer,ityaptin/ceilometer,sileht/aodh,redhat-openstack/ceilometer,chungg/aodh,idegtiarov/ceilometer,ityaptin/ceilometer