Install ldappool when LDAP is enabled

Keystone expects ldappool to be installed when using the LDAP backend.

Change-Id: I35e6585ec8416153ed6d0c6fa8c10c992bf0de6d
Closes-Bug: #1373750

lib/ldap

@@ -139,6 +139,8 @@ function install_ldap {
         sudo ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/inetorgperson.ldif
     fi
 
+    pip_install ldappool
+
     rm -rf $tmp_ldap_dir
 }