Mark cluster's IPs as externally reserved

Mark cluster's IPs as externally reserved

Currently, upon network creation, nodes' and master's IPs
are reserved in the pool.

This leads to pool reservations ('X' in map) that cannot be changed
afterwards, although they may need to (e.g. in case of node
removal/modification). Also, these IPs are not actually handled by
Ganeti in the sense that they are not assigned to the cluster's VMs.

Thus, we should mark the as externally reserved and not just reserved.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>