Don't try to drop FK constraints for sqlite
Don't try to drop FK constraints for sqlite

Unit tests were failing with SQLAlchemy 1.1.0 because the tests try to
drop foreign keys which sqlite doesn't support.

This is because SQLAlchemy 1.1.0 changed to return foreign keys on
tables rather than return no foreign keys.

Change-Id: Ic6fad5ff16e09b4fb8acfe2f670a6b428fd38905
Closes-Bug: 1467008

ajayaa/keystone,takeshineshiro/keystone,roopali8/keystone,idjaw/keystone,rajalokan/keystone,jonnary/keystone,takeshineshiro/keystone,vivekdhayaal/keystone,jonnary/keystone,ilay09/keystone,cernops/keystone,himanshu-setia/keystone,idjaw/keystone,openstack/keystone,ajayaa/keystone,dstanek/keystone,maestro-hybrid-cloud/keystone,maestro-hybrid-cloud/keystone,vivekdhayaal/keystone,dims/keystone,ilay09/keystone,vivekdhayaal/keystone,ilay09/keystone,roopali8/keystone,klmitch/keystone,dims/keystone,dstanek/keystone,openstack/keystone,klmitch/keystone,openstack/keystone,cernops/keystone,jamielennox/keystone,jamielennox/keystone,mahak/keystone,mahak/keystone,dstanek/keystone,himanshu-setia/keystone,mahak/keystone,rajalokan/keystone,rajalokan/keystone