Allow Geo tracking DB user password to be set

With this change, when this is set in /etc/gitlab/gitlab.rb,

geo_postgresql['sql_user_password'] = 'aca53b855d02a1496e2e6d65fe3e39e0'

on reconfigure, the tracking DB user's password gets set to md5aca53b855d02a1496e2e6d65fe3e39e0 which allows the DB to accept username gitlab_geo and password mypassword.

Resolves #4134 (closed).

Side issue: I also tried commenting out the setting to see if it sets the password back to NULL but it does not. It appears to me like https://gitlab.com/gitlab-org/omnibus-gitlab/blob/84852be7/files/gitlab-cookbooks/postgresql/resources/user.rb#L21 may not work as intended? => #4135 (closed)