About mysql : Building-up-a-centralized-authlogin-system-using-ubuntu-libpam-mysql

I’m trying to build up a centralized authentication system in order to let users login into multiple ubuntu server.
Currently, the implemented solution is based on a LDAP server and It works good but I just want to migrate to a different one:
I want to store credentials (username + password OR username + rsa key) into a remote mysql database. Once an user try to ssh into a server (“ssh user_name@ip_target_server”), the server should check whether exists an entry with the specified username on the remote mysql or not and then check the password / rsa correctness.
On success, the target server should create a new user (if not exists).
I’m quite sure that this system should leverage on PAM module like pam-mysql or pam-nss, I tried different guides and tutorials around the web but nothing helped because typically they suggest to create a user on the target manually (I’d prefer automatizing this aspect).
I’m not fluent with PAM-modules so can someone, please, give me any advice? Is there a possible solution or am I on the wrong way?