Warns a user when specifying a VTEP name.

This patch introduces ReadOnlySingleAttr, which throws a newly defined
UserException when a user attempts to set a value to a read-only
attribute, E.g. a VTEP name. When CLI receives a command that attempts
to set a read-only attribute, it'll abort the command execution and go
back to the command line evaluation loop.

Fix MN-2510
it's awkward that at ReadOnlySingleAttr takes a 'setter'.