rhel7 and centos7 images are both only available on x86_64 arch.
if $ARCH is misconfigured, some strange error will happen during the build.
For example, DIB will try to install EPEL i386 on the 64bit root system.
For the record, tripleo-incubator $NODE_ARCH default value is i386. The
problem will happend as soon as the default value is used with one of
these root elements.
This commit ensure the $ARCH is set to amd64 as soon as the centos7 or
rhel7 root element are used.
Change-Id: Ie41fa2da48eac6bf89b96cfa137c0f572dae6734
725f91078e