Flush tenant membership deletion before user deletion

This ensures that "smartness" of sqlalchemy does not come in the way
of user deletion.

See also https://review.openstack.org/#/c/7353 and upstream bug 998137,
and analogous change for projects done for altai-19:
http://khar.ci-cd.altai-dev.griddynamics.net/gerrit/#/c/127/

Change-Id: I914dabd26ca86efa481abe245ee3066ba519532f