Include CephFS mount options from Manila as posix mount options.

[tomclark0]

This change extracts the __mount_options field from the metadata of Manila shares and includes it in the mount_options variable.
This field is expected to be populated only with CephFS shares.

Key functionality:

• Pass Manila provided mount options as mount_options
• Support for mounting CephFS when multiple backends or multiple CephFS pools are in-use.

This allows the handling of CephFS shares in Manila, where multiple CephFS pools are used within a single Ceph cluster.

[sjpb]

Defaults are documented in https://github.com/stackhpc/ansible-role-os-manila-mount?tab=readme-ov-file#role-variables, the behaviour of defaults also coming from the share should be documented.

[tomclark0]

Thanks @sjpb.
I've changed approach and removed the user facing var as a duplicate.
Any manila provided mount options either manually set via openstack share set --property "__mount_options=" share_id or added due to different backends are honoured on a per-share basis.

[sjpb]

LGTM