Add pre-checker to make sure custom postgresql repository configured

[SandakovMM]

When a custom PostgreSQL version newer than 10 is installed, but the repository file is missing, centos2alma will be unable to re-install PostgreSQL on the other side, leaving the server non-functional.
Unfortunately, centos2alma cannot determine the exact repository from which PostgreSQL was originally installed. We can only add a pre-check to ensure the repository file can be found at /etc/yum.repo.d