Set and export DIB_RELEASE for centos7

The default value was set in the centos7 element, but not
exported, which caused issues in rpm-distro. Also changed
a test in rpm-distro to only check for DIB_RELEASE > 22
if it's fedora.

Closes-Bug: #1477172

Change-Id: Ib6f4227411c2e8f1965c3b78bc318512c59a7876

elements/centos7/environment.d/10-centos7-distro-name.bash

@@ -1 +1,2 @@
+export DIB_RELEASE=GenericCloud
 export DISTRO_NAME=centos7

elements/rpm-distro/pre-install.d/01-override-yum-arch

@@ -25,7 +25,7 @@ else
     exit 1
 fi
 
-if [ $DIB_RELEASE -ge 22 ]; then
+if [[ $DISTRO_NAME == "fedora" && $DIB_RELEASE -ge 22 ]]; then
     mkdir -p /etc/dnf/vars
     echo $basearch > /etc/dnf/vars/basearch
     echo $arch > /etc/dnf/vars/basearch

elements/yum/post-install.d/99-reset-yum-conf

@@ -6,7 +6,7 @@ fi
 set -eu
 set -o pipefail
 
-if [ $DIB_RELEASE -ge 22 ]; then
+if [[ $DISTRO_NAME == "fedora" && $DIB_RELEASE -ge 22 ]]; then
     cfg=/etc/dnf/dnf.conf
 else
     cfg=/etc/yum.conf

elements/yum/pre-install.d/01-yum-keepcache

@@ -6,7 +6,7 @@ fi
 set -eu
 set -o pipefail
 
-if [ $DIB_RELEASE -ge 22 ]; then
+if [[ $DISTRO_NAME == "fedora" && $DIB_RELEASE -ge 22 ]]; then
     # dnf is slightly different; doesn't have these by default
     cfg=/etc/dnf/dnf.conf
     echo "keepcache=1" >> $cfg