jenkins-common.sh: add image_suffix_is_master()

Since recently, we also have a new IMAGE_SUFFIX '2021q1-centos8'.
We cannot rely on image_suffix_is_latest() anymore, because this
prefix does not qualify as 'latest'.  If one needs to enable a
feature that is only available in master, then here is a way:

  if ! image_suffix_is_master; then
      # Patch the configuration files
  fi

Change-Id: I65df66cd2e14934bed8114618d6ffad7831d1025

jenkins-common.sh

@@ -343,6 +343,14 @@ image_suffix_is_latest() {
 	esac
 }
 
+# Check if IMAGE_SUFFIX starts with "master" (e.g. "master-centos8")
+image_suffix_is_master() {
+	case "$IMAGE_SUFFIX" in
+	master*) return 0 ;;
+	*) return 1 ;;
+	esac
+}
+
 set -x
 
 # non-jenkins execution: assume local user name