use context manager from neutron-lib
use context manager from neutron-lib

The neutron.db.api.context_manager already references neutron-lib's
context manager; so consumers of it are already using neutron-lib. This
patch switches neutron's references to the context_manager over to
use neutron-lib's directly rather than that in neutron.db.api.

NeutronLibImpact

Change-Id: I6bd6283b4bcc018d1e0fcd0972ec4a4278b24f3e

stackforge/tricircle,openstack/tricircle,stackforge/tricircle,openstack/tricircle