Add function is_plugin_enabled

Add a function which tests if a plugin has been enabled with
enable_plugin. This is helpful if two co-ordinating projects want to run
specific setup in devstack in one only if the other is enabled.

Change-Id: Ibf113755595b19d028374cdc1c86e19b5170be4f

Author: omeranson

functions-common

@@ -1882,7 +1882,7 @@ function enable_plugin {
     local name=$1
     local url=$2
     local branch=${3:-master}
-    if [[ ",${DEVSTACK_PLUGINS}," =~ ,${name}, ]]; then
+    if is_plugin_enabled $name; then
         die $LINENO "Plugin attempted to be enabled twice: ${name} ${url} ${branch}"
     fi
     DEVSTACK_PLUGINS+=",$name"
@@ -1891,6 +1891,19 @@ function enable_plugin {
     GITBRANCH[$name]=$branch
 }
 
+# is_plugin_enabled <name>
+#
+# Check if the plugin was enabled, e.g. using enable_plugin
+#
+# ``name`` The name with which the plugin was enabled
+function is_plugin_enabled {
+    local name=$1
+    if [[ ",${DEVSTACK_PLUGINS}," =~ ",${name}," ]]; then
+        return 0
+    fi
+    return 1
+}
+
 # fetch_plugins
 #
 # clones all plugins