mgr/dashboard: consume mgr/nfs via mgr.remote()

mgr/dashboard: consume mgr/nfs via mgr.remote()

Stop using the dashboard version of the Ganesha config classes; consume
mgr/nfs instead via remote().

mgr/nfs/export: return Export from _apply_export

Future callers will want this.

mgr/nfs: new module methods for dashboard consumption

Add some new methods that are easy for the dashboard API to consume.  These
are very similar to the CLI methods but do now have the @CLICommand and
related decorators, and have slightly different interfaces (e.g., returning
the created/modified Export dict).

mgr/dashboard: remove old ganesha code (and tests)

Fixes: https://tracker.ceph.com/issues/46493
Signed-off-by: Sage Weil <sage@newdream.net>