Fixes Interface IP and Netmask Defaults

Previously, ip_address and netmask parameters were required.
This causes problems with interfaces that are set to manual for
bridging OVS traffic.

ip_address and netmask have been changed to undef so an IP
address and network mask is not required to be configured on an
interface.
Merge pull request #30 from CiscoSystems/interface_defaults_fix

Fixes Interface IP and Netmask Defaults