postgresql: only pass username and password if they are set
postgresql: only pass username and password if they are set

Passing None makes psycopg2 >= 2.7 use the string "None" as a role rather than
nothing like it used to do.

Change-Id: Idf575930ea363a7e7e613ebdca4595a9ca8f7389

openstack/tooz,openstack/tooz