no_ip() rasies an exception when subnet is used as an argument

[Ante Karamatić]

is_ip() in charmhelpers/contrib/network/ip.py returns True if value it an IP, and False if it's not an IP. It does that by returning False when AddrFormatError is raised. This works fine for nonsense and interfaces:

>>> netaddr.IPAddress("[eth0]")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/netaddr/ip/__init__.py", line 306, in __init__
    'address from %r' % addr)
netaddr.core.AddrFormatError: failed to detect a valid IP address from '[eth0]'
>>> netaddr.IPAddress("nonsense")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/netaddr/ip/__init__.py", line 306, in __init__
    'address from %r' % addr)
netaddr.core.AddrFormatError: failed to detect a valid IP address from 'nonsense'

but it fails for subnets:

>>> netaddr.IPAddress("192.168.0.100/16")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/netaddr/ip/__init__.py", line 280, in __init__
    % self.__class__.__name__)
ValueError: IPAddress() does not support netmasks or subnet prefixes! See documentation for details.

This is because subnet, for some reason, raises ValueError.