Do not allow using METADATA as the group name on the CP data structures

[mdumandag]

METADATA is a special group name dedicated to the internal CP group that is used for maintaining CP groups and members. As of now, when you try to create a CP data structure with the METADATA as the group name, you receive an error from the server-side. However, it would be good if we can fail fast on the client-side.

So, a code like this should fail on the client-side.

import hazelcast

client = hazelcast.HazelcastClient()

view_counter = client.cp_subsystem.get_atomic_long("views@METADATA")