postgresql: fix db-fas01.stg breakage from playbook runs
    
    In prod db-fas01 is rhel7 and uses postgresql 9.6.
    In staging db-fas01.stg is rhel8, and should also use postgresql 9.6,
    but we were blanket making rhel8 hosts use postgresql 12.
    We could drop this by reinstalling db-fas01.stg with rhel7, or waiting
    until we finally kill fas2 and just setting them both to use postgresql
    12.
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>