Cirrus: Skip rebuilding images unless instructed

Given frequent merges, it doesn't make sense to rebuild the VM testing
images every time.  Instead, monitor the PR title and description for
a magic string, only triggering builds on a match:

***CIRRUS: REBUILD IMAGES***

Signed-off-by: Chris Evich <cevich@redhat.com>

contrib/cirrus/build_vm_images.sh

@@ -22,9 +22,7 @@ SCRIPT_BASE $SCRIPT_BASE
 PACKER_BASE $PACKER_BASE
 "
 
-# TODO: Skip building images if $CIRRUS_BRANCH =~ "master" and
-# commit message of $CIRRUS_CHANGE_IN_REPO contains a magic word
-# produced by 'commit_and_create_upstream_pr.sh' script (see .cirrus.yml)
+require_regex '\*\*\*\s*CIRRUS:\s*REBUILD\s*IMAGES\s*\*\*\*' 'Not re-building VM images'
 
 show_env_vars
 

contrib/cirrus/lib.sh

@@ -117,6 +117,22 @@ cdsudo() {
     sudo --preserve-env=GOPATH --non-interactive bash -c "$CMD"
 }
 
+# Skip a build if $1 does not match in the PR Title/Description with message $2
+require_regex() {
+    req_env_var "
+        CIRRUS_CHANGE_MESSAGE $CIRRUS_CHANGE_MESSAGE
+        1 $1
+        2 $2
+    "
+    regex="$1"
+    msg="$2"
+    if ! echo "$CIRRUS_CHANGE_MESSAGE" | egrep -q "$regex"
+    then
+        echo "***** The PR Title/Description did not match the regular expression: $MAGIC_RE"
+        echo "***** $msg"
+        exit 0
+    fi
+}
 
 # Helper/wrapper script to only show stderr/stdout on non-zero exit
 install_ooe() {