Merge pull request #514 from FollowKenny/fix_ipabackup_var

change variable in get_ipabackup_dir.yml and update README.md

roles/ipabackup/tasks/get_ipabackup_dir.yml

 ---
 - name: Get IPA_BACKUP_DIR dir from ipaplatform
-  command: "{{ ansible_playbook_python }}"
+  command: "{{ ansible_python_interpreter | default('/usr/bin/python') }}"
   args:
     stdin: |
       from ipaplatform.paths import paths