Update Ceilometer alembic requirement to 0.6.0 version

Our ceilometer/storage/sqlalchemy/alembic/versions/b6ae66d05e3_remove_extra_indexes.py call a new drop_constraint method with type_ argument,
but this new method argument is invovled from alembic 0.5.0 version, Jenkins is using alembic 0.6.0 version

Fixed bug: 1221467

Change-Id: I78f1e538c39fe4c972f21aa0fe77d3c82087176a
Merge "Update Ceilometer alembic requirement to 0.6.0 version"