Add support for setting db_sslca in gitlab.rb

Fixes https://gitlab.com/gitlab-org/omnibus-gitlab/issues/789

db_sslca and db_sslrootcert mean the same thing, but are used by different activerecord adapters Added support for setting either one for setting the ca cert

Not currently adding db_sslca cert to the gitlab.rb.template because having two settings for the same thing is confusing