Demote personalities from core of API v3 as extensions os-personality

This patch move 'personality' from core into extensions.
* demote personality the attribute of server from servers
* add corresponding tests
* move some tests about personality from test_servers to test_personality

Partially implements blueprint nova-v3-api

Change-Id: I62254a94476271b1d8bcc206dfda1236f155960b
Merge "Demote personalities from core of API v3 as extensions os-personality"