Bad `create monitor` command leaves a ghost monitor

When the creation of a monitor fails at runtime, if another monitor already monitors one of the servers given in the initial servers relationship, the "unclaimed" servers that aren't monitored by anything are permanenty linked with the monitor being created. This causes further attempts to use them to fail with an error that states they are already monitored by another monitor.

The following commands will reproduce the problem:

maxctrl create server server1 127.0.0.1 3000

maxctrl create monitor MariaDB-Monitor mariadbmon user=maxuser password=maxpwd --servers server1 server1