Add pg_baseclone script

That's follow up on request for improvement in regards to tooling helping with database replication recovery process. Hoping you'll find it helpful/useful, enjoy!

New script pg_baseclone offers similar functionality as the current script (clones database data from primary node to local node using pg_basebackup), only runs in fail-early regime (-eu, pipefail)) and makes extra effort to verify prerequisites before proceeding:

• verifies status of the cluster and confirms status of the local node (inspects crm status output and iptables rules)
• confirms psql access to the primary node
• (optional) goes extra mile to verify status of the primary node using readiness healthcheck

In a wee test environment new script worked fine, still in production caution is advised.