Install pymemcache explicitly

The pymemcache package is needed by ironic-inspector-api process
when running in non-standalone mode.
Since devstack doesn't install test-requirements anymore, we need
to force installation in the devstack plugin before the inspector
processes start.

Change-Id: Iff0de5eb99db4a3dcb0d5ff09b908484f9532492

devstack/plugin.sh

@@ -125,6 +125,10 @@ function install_inspector {
 
     if [[ "$IRONIC_INSPECTOR_STANDALONE" == "False" ]]; then
         install_apache_wsgi
+        # NOTE(rpittau) since devstack doesn't install test-requirements
+        # anymore we need to install dependencies for drivers before
+        # starting inspector services
+        pip_install_gr pymemcache
     fi
 }