Warn about use of overlapping ips in config file

Fix bug 1055822

Add an explicity user in the configuration file regarding consequences of
enabling overlapping IPs when Quantum is used with nova security groups
and/or metadata.

Change-Id: Ifa67216fc7479a7858c0653422bf5baab58fb350

etc/quantum.conf

@@ -45,6 +45,8 @@ api_paste_config = api-paste.ini
 # Enable or disable bulk create/update/delete operations
 # allow_bulk = True
 # Enable or disable overlapping IPs for subnets
+# Attention: the following parameter MUST be set to False if Quantum is
+# being used in conjunction with nova security groups and/or metadata service.
 # allow_overlapping_ips = False
 
 # RPC configuration options. Defined in rpc __init__