Not properly handling ipv6 on contrib.network.ip.get_address_in_network

[Jorge Niedbalski]

[Description]

While investigating the root cause of https://bugs.launchpad.net/charm-ceph-fs/+bug/1659338
I discovered a couple of issues on the function get_address_in_network
when using ipv6 addresses.

1) ::1 must be skipped when iterating over the list of available networks
as this is the loopback address.

unit-ceph-fs-4[2718]: 2017-03-06 20:32:16 INFO unit.ceph-fs/4.ceph-mds-relation-changed logger.go:40 netaddr.core.AddrFormatError: '::1' is not a valid IPv4 address string!

2) IPNetwork doesn't support netmasks for ipv6 addresses,
instead a prefix should be used.

unit-ceph-fs-4[2718]: 2017-03-09 21:25:03 INFO unit.ceph-fs/4.ceph-mds-relation-changed logger.go:40 ValueError: IPAddress() does not support netmasks or subnet prefixes! See documentation for details. (Full stacktrace: http://pastebin.ubuntu.com/24148408/)