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. VTEP name and description. 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