Remove unused physical_network parameter in tripleo-image-elements

It was a leftover after `a7d02f115c71a00b6c4a1ffaf968481d6e0afec0` cleanup.

Change-Id: I76aaa2c1f4732d72d28943b44d0760089dda6dd0
Closes-Bug: 1345086
Merge "Remove unused physical_network parameter in tripleo-image-elements"