standby-clone: Allow cloning of standby server with pg_basebackup

standby-clone: Allow cloning of standby server with pg_basebackup

Right now the role assumes you have a base backup available with
Barman. However if you don't have an initial barman backup you might
want to clone the primary server directly to setup your standby
server.

This PR adds a new `primary.pg_basebackup` option to the cluster
configuration which if enabled (set to `true`) will create a
`/root/standby-clone-{{ postgres_version }}-{{ postgres_cluster_name }}.sh`
script on the standby server which helps to initialise a standby
server.

⚠️ Breaking change: the current role behavior which creates a cloning
script fetching the initial backup from barman will not be
enabled by default anymore. You will need to add the new
`primary.restore_barman_directory` option in your role
configuration to do so. ⚠️