Added variable osa_backup_command so that mariabackup or innobackupex can be used for backing up Galera depending on what is supported on the source database cluster

Updated backup to used osa_backup_command variable which can be chosen from the default/main.yml to allow support for innobackupex or mariabackup

Removed extra quotes from osa_backup_command innobackupex line

Updated readme to explain reason for forking

Change the default backup command from innobackupex to mariabackup

Turns out the ansible_date_time uses a cache date and time rather than actual. With openstack-ansible this can be days out of date. This update ensures the actual date and time is used as part of the mariabackup dir