CephFS driver must stop using DBUS to communicate with nfs-ganesha

The CephFS driver in manila currently uses "dbus-send" to send signals to nfs-ganesha in order to create, update and delete NFS exports. This approach has some pitfalls:

1) Service deployment is constrained: the manila-share service (where the cephfs driver runs) has to be able to have access to the dbus socket that the nfs-ganesha service listens on. When they're running on the same host, this isn't a problem. However, when these services are containerized or when they're running on different hosts, the driver requires (a) the dbus socket file to be shared between two containers on the same host; or (b) root access to the host where the nfs-ganesha service is running (because the driver doesn't request elevated privileges when issuing dbus-send over ssh)*

2) Ceph config file cannot be customized. This is a bug in nfs-ganesha: https://bugzilla.redhat.com/show_bug.cgi?id=1901143 ; however manifests in the way the manila cephfs driver creates its exports - these exports are written as separate export files intimated via dbus.

3) Mutiple instances of nfs-ganesha cannot be updated simultaneously - this limits using nfs-ganesha in an active/active configuration. We could send separate dbus-send commands however, the manila driver would have to own the synchronization, error handling and retries-on-failure.

So better direction here would be to use the "watch_url" mechanism where any number of ganesha instances (and others too!) can be made to watch a rados url, and sending a notification would signal ganesha daemon/s to reload configuration and apply exports.

1) (b) can be resolved by creating a user that has SSH access and permissions to use dbus-send via switching the policy context, however, the nfs-ganesha team had turned down this request: https://github.com/nfs-ganesha/nfs-ganesha/issues/219