Make live LDAP user DN match the default from devstack

The default LDAP user DN in devstack uses the "cn" attribute in
the RDN.  The live LDAP test default configuration is supposed to
match the default devstack config, but the RDN is incorrectly
using the "dc" attribute.  This patch corrects the live LDAP
default config to match devstack.

Change-Id: Iee49606526047105b4e60142af32b4ec81a19444
Closes-Bug: 1280509
Merge "Make live LDAP user DN match the default from devstack"