force arch amd64 for EL7 elements

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

elements/centos7/root.d/10-centos7-cloud-image

@@ -11,6 +11,9 @@ set -o pipefail
 
 if [ 'amd64' = "$ARCH" ] ; then
     ARCH="x86_64"
+else
+    echo 'centos7 root element only support the amd64 $ARCH value.'
+    exit 1
 fi
 
 DIB_LOCAL_IMAGE=${DIB_LOCAL_IMAGE:-}

elements/rhel7/root.d/10-rhel7-cloud-image

@@ -11,6 +11,9 @@ set -o pipefail
 
 if [ 'amd64' = "$ARCH" ] ; then
     ARCH="x86_64"
+else
+    echo 'rhel7 root element only support the amd64 $ARCH value.'
+    exit 1
 fi
 
 DIB_LOCAL_IMAGE=${DIB_LOCAL_IMAGE:-""}