Don't use sudo to discover ipv4 address.

Running unit tests on a plane (with no network) and I recently
discovered that the Nova unit tests were prompting me for sudo.

A quick look showed the following command being executed:

 nova-rootwrap /etc/nova/rootwrap.conf ip -f inet -o route show

This patch updates utils.get_my_ipv4_address() and
the private _get_ipv4_address_for_interface function so they run
commands without using sudo.

Change-Id: Id9c33c9383a9f5b2e1fce1b0f5afcc2a27a271c6
Merge "Don't use sudo to discover ipv4 address."