pgbouncer unix_socket_dir set to /tmp by default

I noticed that the omnibus pgbouncer leaves the unix_socket_dir config parameter blank, which puts the socket file in /tmp.

Note that this is important because the gitlab-psql user can connect to the pgbouncer instance with no password:

sudo -u gitlab-psql /opt/gitlab/embedded/bin/psql -p 6432 -h /tmp pgbouncer -U pgbouncer

We should probably put this in /var/opt/gitlab/pgbouncer or somewhere managed by omnibus.

/cc: @jtevnan, @ibaum