cyclades: Exclude drained networks in backend-list

Exclude drained networks when counting available public IPs of
each backend, in the 'backend-list' management command.
cyclades: Fix available IPs in subnet-inspect

Available IPs in subnet-inspect command should not contain the
externally reserved IPs.
cyclades: Optionally inspect network in backends

Add '--backends' option in 'network-inspect' command to handle
whether to inspect the state of network in all Ganeti backends.
cyclades: Include IP stats in network-inspect

Include the number of free and total IPv4 addresses in the output
of 'network-inspect' management command.