Add os-net-config dependency

I5f51ecb5c64fc9c13eabc16b2b43b5dda1b38d65 in tripleo-image-elements
added a dependency on the os-net-config element which installs
the os-net-config package but global requirements is missing
this package.

Change-Id: I62e19ec92fbcfa3c029aeecdeed08c1e58f19d7f
Merge "Add os-net-config dependency"