Simplify sqlalchemy imports in driver
Simplify sqlalchemy imports in driver

This greatly simplifies the sqlalchemy-related imports in
glance.db.sqlalchemy.api. Related to bp refactor-db-layer.

Change-Id: I60d39baee420c68efd2f00bbd13162d1b78d9c94

stevelle/glance,ozamiatin/glance,rajalokan/glance,JioCloud/glance,citrix-openstack-build/glance,tanglei528/glance,ozamiatin/glance,sigmavirus24/glance,jumpstarter-io/glance,akash1808/glance,citrix-openstack-build/glance,akash1808/glance,cloudbau/glance,klmitch/glance,darren-wang/gl,vuntz/glance,wkoathp/glance,stevelle/glance,takeshineshiro/glance,openstack/glance,redhat-openstack/glance,ntt-sic/glance,paramite/glance,scripnichenko/glance,SUSE-Cloud/glance,openstack/glance,cloudbau/glance,ntt-sic/glance,dims/glance,vuntz/glance,darren-wang/gl,kfwang/Glance-OVA-OVF,paramite/glance,jumpstarter-io/glance,rickerc/glance_audit,rajalokan/glance,kfwang/Glance-OVA-OVF,sigmavirus24/glance,saeki-masaki/glance,saeki-masaki/glance,dims/glance,SUSE-Cloud/glance,wkoathp/glance,tanglei528/glance,rickerc/glance_audit,openstack/glance,klmitch/glance,redhat-openstack/glance,scripnichenko/glance,JioCloud/glance,takeshineshiro/glance