Added helper functions in netutils and related constants

Added the following functions to netutils:
- IsValidInterface
- GetInterfaceIpAddresses
- _GetIpAddressesFromIpOutput

Added the following static methods to netutils.IPAddress:
- GetAddressFamilyFromVersion
- GetVersionFromAddressFamily

Added unit tests for the new methods in netutils.IPAddress, for the IP
address search regex and for GetInterfaceIpAddresses

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>