Allow release id to be specified with fedora

This resolves the fedora image build failure below:
http://logs.openstack.org/periodic/git.openstack.org/openstack/octavia/ \
master/publish-openstack-octavia-amphora-image-fedora/6be692a/job-output.txt.gz

Change-Id: Ide8ebbc6264afb2f33cf2d6803aaf21631449424

diskimage-create/diskimage-create.sh

@@ -194,6 +194,8 @@ if [ "$AMP_BASEOS" = "ubuntu-minimal" ]; then
     export DIB_RELEASE=${AMP_DIB_RELEASE:-"xenial"}
 elif [ "${AMP_BASEOS}" = "centos" ] || [ "${AMP_BASEOS}" = "rhel" ]; then
     export DIB_RELEASE=${AMP_DIB_RELEASE:-"7"}
+elif [ "${AMP_BASEOS}" = "fedora" ]; then
+    export DIB_RELEASE=${AMP_DIB_RELEASE:-"28"}
 fi
 
 AMP_OUTPUTFILENAME=${AMP_OUTPUTFILENAME:-"$PWD/amphora-x64-haproxy"}

zuul.d/jobs.yaml

@@ -103,6 +103,7 @@
     vars:
         amphora_os: fedora
         amphora_image_size: 3
+        amphora_os_release: 28
 
 - job:
     name: octavia-grenade