Implement MaxCtrl

Implement MaxCtrl - the MaxScale administration tool that uses the new REST-API.

• Everything that is available and modifiable via the current maxadmin should be available and modifiable via maxctrl.
• Maxctrl must be able to connect to more MaxScale instances than one.
• Maxctlr must be able to make modifications simultaneously to several MaxScale instances at once (some measures should be taken to reduce the risk that modifications inadvertently are made when one of the MaxScale instances is down or unreachable, e.g. first ping both before actually performing the command).
• Maxctrl must be able to visualize the data from several MaxScale instances in a manner that makes it straightforward for a user to spot discrepancies.
• Even if Maxctrl is connected to multiple MaxScale instances it must be possible to explicitly make modifications to a specific instance (e.g. to fix noticed discrepancies).