New functionalities: 
- make-co-master: promotes a slave to eb co-master with its master
- detach-slave: detaches a slave from its master (breaks replication)
The above allow for replacing a topology's master.

cli: added make-co-master, detach-slave
api: added make-co-master, detach-slave 

web:
- instance dialog supports "detach slave" action
- better error visualization for a master which is a co-master
- Drag & drop support for making a co-master. It is now possible to drag
a master onto one of its immedite slaves.