Add rhel7-rdo-icehouse element

This element is for building RHEL7 images using rdo packages from
the Fedora Icehouse repository.

This patch relies on the following patches from tripleo-image-elements:
I8c35d55e7cffb358613436d87ee67e6484533bbb
I092f5f2326c424694cf34578b7fca7e2a3f4d4d5

Change-Id: I2bd9751e2678a1388fafde92ab247fab301575d8
Merge "Add rhel7-rdo-icehouse element"