Check existence of rhel rpm key

The key 'RPM-GPG-KEY-redhat-release' may not exist in image
e.g. when you use non-default image (for example, if you build CentOS
image using rhel element).

Fixes bug #1252697

Change-Id: Iae642abb7a7f76c5528750dafedd4fd4bda56fe2

elements/rhel/bin/install-packages

@@ -33,6 +33,8 @@ do
   fi
 done
 if [ -n "$WHITELIST" ]; then
+  if [ -f /etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release ]; then
     rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release
+  fi
   yum -y install $(map-packages $WHITELIST)
 fi