update share_type's name to an exist share_type name, Postgres database will error, but not mysql.

[haixin]

manila type-create Example_1 --snapshot_support False False
manila type-create Example_2 --snapshot_support False False
manila type-update Example_2 --name "Example_1"

if the datebase is Postgres database will get DB error follow:
Error traceback: http://paste.openstack.org/show/774664/

but mysql database does not has that error,

The following code affects the problem described above:
https://review.opendev.org/#/c/669651/24/manila/db/sqlalchemy/api.py@3978.
if change ~model.deleted to model.deleted != "False".it will lead to unit test failed.
https://ce298eee25173aa90c34-ca4b089b4b338eb03b97f6a00e3061e2.ssl.cf2.rackcdn.com/669651/25/check/openstack-tox-py27/49413f1/job-output.txt

[Goutham Pacha Ravi (gouthamr)]

This bug was fixed by enforcing the uniqueness constraint added to the ShareTypes model:
https://review.opendev.org/#/c/669651/24..29/manila/db/sqlalchemy/api.py
https://review.opendev.org/#/c/669651/26/manila/db/sqlalchemy/api.py@3978