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
Added NFS service switch on top NFS share page
Update alert message. #483
Merge pull request #493 from schakrava/issue#483_unresolvable_nfs_client

Validate host string input in nfs export refresh.