Validate host string input in nfs export refresh.

Fixes #483
Fixes #376
Merge branch 'issue#483_unresolvable_nfs_client' of git://github.com/schakrava/rockstor-core into schakrava-issue#483_unresolvable_nfs_client
Add switch in low level snapshot create code so they can be created either readonly or readwrite.
Fixes #492.

clone is buggy due to which a clone of a snapshot is really just creating a clone of the share to which that snapshot belongs to. Fix this bug.
Fixes #492

Cloning of a share should call the clone helper with right signature.
Fixes #492

Snapshots can be created in readonly or readwrite mode. Enable this switch in the API.
remove very chatty debug logging
Merge branch 'issue#492_snapshot_rollback' of git://github.com/schakrava/rockstor-core into schakrava-issue#492_snapshot_rollback
Add read-field to add snapshot page.

Fixes #492
#492. Use writable through out instead of readonly for consistency.
Merge branch 'issue#492_snapshot_rollback' of git://github.com/schakrava/rockstor-core into schakrava-issue#492_snapshot_rollback
Add writable field to add snapshot form and change roll back functionality ad per the writable value.

Fixes: #492
Change form layout for snapshot create form

Fixes: #492
Add writable column in snapshot table list.

Fixes: #492
Change the writable value from boolean to string. #492
Don't give the option to clone readonly snapshots. #492
Accept writable as a string and convert to boolean. #492
Improve tooltips and misc messages. #492
Added NFS service switch on top NFS share page
Update alert message. #483