Provide option to change backend

[wsidl]

OS: CentOS 7 Ansible: 2.7 Python: 3.7

Been testing a new configuration and using this role has gotten me closer to anything else. My only issue is the default "bdb" backend has been causing a number of issues, especially on the last task ("add the base domain").

After a few hours of searching, decided to try changing the backend to "mdb" and everything works well. Re-running this role with the "slapd.conf.j2" file updated to use a different backend has resolved the issue.