Add add version check command

Closes https://git.autonomic.zone/coop-cloud/abra/issues/108.

abra

@@ -561,6 +561,11 @@ output_version_summary() {
     done
   done
 
+  if [[ -n "$IS_VERSION_CHECK" ]] && [[ "$IS_VERSION_CHECK" == "true" ]]; then
+    debug "Detected version check (without deploy), bailing out..."
+    exit 0
+  fi
+
   if [[ $IS_AN_UPDATE == "true" ]] && [[ $FORCE_DEPLOY != "true" ]]; then
     require_consent_for_update
   else
@@ -1142,9 +1147,19 @@ sub_app_config (){
 }
 
 ###### .. app version
+help_app_version (){
+  echo "abra [options] app <app> version
+
+Show versions of the app that are currently deployed"
+}
+
 sub_app_version (){
-  # shellcheck disable=SC2016
-  echo '`abra app <app> version` command working correctly!'
+  require_yq
+
+  IS_VERSION_CHECK="true"
+
+  echo "Version overview:"
+  output_version_summary
 }
 
 ###### .. app check