Undeprecate enable_security_group parameter

Introduced by commit f87e7d964c19cc2be33226df66f0c823af993d49
in Neutron 2014.1.rc1, this config is far from deprecated.

Restore it with proper default value.

Change-Id: If74658dbdb0fc1961a664df7484ebc2d1b0a118b
Merge "Undeprecate enable_security_group parameter"