ipmanager: add new options for DHCP clusters => clustermode & clustermaster
* clustermode permit to define two modes for ISC-DHCPd master/slave and load-balance
* clustermaster elect a master of the cluster
ipmanager: now add Z-Eye service handling for DHCP clusters. Add experimental SQL table for IP ranges
ipmanager: implement range management. We are now near 1.2
ipmanager: now show the ranges on interface
ipmanager: refresh subnet view when modify ranges
ipmanager: fix double cluster view on subnet form
ipmanager: also calculate ranges when create a new reservation
ipmanager: convert all array_push calls to modern $var[] = $value
* also cleanup the code for better reading