PostgreSQL connector should default to "localhost" if no HOST is provided instead of erroring.

[bckohan]

Current Situation

If HOST key is not present in the DATABASES setting dbbackup will error out.

Proposed Actions

It is valid Django that if HOST is not present it is assumed to be localhost.

I propose that if HOST is not present it is assumed to be localhost as the Django docs suggest.

System Information

all systems are affected.

[sur5r]

Actually, the fix is wrong. The Django docs explicetely state that empty host string means unix domain socket connection for postgres. This also reported in #490. So the fallback for unset HOST should be "" and not "localhost".