Add ValidateNetmask and GetClass IPAddress methods

Add the following methods to netutils.IPAddress:
* ValidateNetmask
* GetClassFromIpVersion
* GetClassFromIpFamily

Also, add related tests to the test suite.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>