5308fbdeac split out `zulip::postgresql_client`, and 80ef38757a
made it no longer depend on `zulip::postgresql_common`, but directly
on `zulipconf('postgresql', 'version', undef)`. However, the
installer depended on recognizing `zulip::postgresql_common` in the
list of pulled-in classes to know that we needed to keep the
`postgresql.version` setting in `/etc/zulip.conf`.
Update the installer to also recognize `zulip::postgresql_client` as a
class which tells us to keep `postgresql.version` in our settings.
(cherry picked from commit 22300373d3)
54e221564b