ci: lib: Skip remount if empty limit pattern

Some playbooks apply only to hosts that do not have read-only root
filesystems.  For these, the `rw_limit` pattern will be empty.  The
*Remount R/W* and *Remount R/O* stages should be skipped when this is
the case.

vars/applyConfigPolicy.groovy

@@ -1,4 +1,5 @@
 import groovy.transform.Field
+import org.jenkinsci.plugins.pipeline.modeldefinition.Utils
 
 @Field
 def DOCKER_ARGS = '''\
@@ -52,7 +53,9 @@ def stageKinit() {
 
 
 def stageRemountRW(limit) {
-    stage('Remount R/W') {
+    def STAGE_NAME = 'Remount R/W'
+    stage(STAGE_NAME) {
+        if (limit) {
             ansiblePlaybook \
                 playbook: 'remount.yml',
                 limit: limit,
@@ -61,6 +64,9 @@ def stageRemountRW(limit) {
                 extraVars: [
                     remount_state: 'rw',
                 ]
+        } else {
+            Utils.markStageSkippedForConditional(STAGE_NAME)
+        }
     }
 }
 
@@ -80,13 +86,18 @@ def generateStages(stages) {
 }
 
 def stageRemountRO(limit) {
-    stage('Remount R/O') {
+    def STAGE_NAME = 'Remount R/W'
+    stage(STAGE_NAME) {
+        if (limit) {
             ansiblePlaybook \
                 playbook: 'remount.yml',
                 limit: limit + ':!rw-root',
                 become: true,
                 vaultCredentialsId: 'ansible-vault',
                 extras: '--diff'
+        } else {
+            Utils.markStageSkippedForConditional(STAGE_NAME)
+        }
     }
 }