Use 'replica' instead of 'slave'

Replace the mysql-specific term 'slave' with the more general term
'replica'.

In order to merge properly the client currently handles both 'slave' and
'replica' in certain cases. The 'slave' support will be removed completely
once the trove runtime is updated to be in sync.

Change-Id: Id22d18a84a4ac104ff8af09959e8eb2ae0102e97
Co-Authored-By: Nikhil Manchanda <SlickNik@gmail.com>
Partial-Bug: 1360310

Conflicts:
	troveclient/v1/instances.py
	troveclient/v1/shell.py