Added get_ip_for_network_interface helper.

[pengale]

Allows a charm to lookup the ip address of a named network interface, or
find a network interface that falls in a cidr range.

Added tests, and fixed things so that tox will actually run test_utils.py.

@juju-solutions/bigdata

[johnsca]

This should be something more like this:

for interface in interfaces:
    for ip_version in (netifaces.AF_INET, netifaces.AF_INET6):
        for address in netifaces.ifaddresses(interface)[ip_version]:
            ip = address['addr']
            ip = getattr(ip, 'decode', lambda e: ip)('utf8')  # force unicode
            ip = ipaddress.ip_address(ip)
            if ip.is_link_local:
                continue
            if ip in ipaddress.ip_network(network_interface):
                return ip

Specifically:

• Loop over all returned addresses, don't just try the first
• Use is_link_local
• I like hasattr(..., 'decode') better than encode / decode, but that's more personal preference