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.

Conflicts:
	manifests/plugins/ml2.pp

Change-Id: If74658dbdb0fc1961a664df7484ebc2d1b0a118b
(cherry picked from commit 5b3abcff91f6d9ec152761848727501ac288d62f)
Merge "Undeprecate enable_security_group parameter" into stable/icehouse