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
Thank you for the pointer, @gontanon. I prepared a separate patch that implements 'writeable'. I'll send a pull request shortly, so please take a look. One concern is that though 'writeable' han't implemented, there are many elements specifying some of their attributes 'writeable=False', therefore implementing this feature now may bring about unexpected breakage somewhere else.