Fix wrong section name "security_group" in sample config files

The section name [security_group] in Sample config files of
ML2 and Big Switch plugins was wrong and it should be
[securitygroup].

Change-Id: I8204b2f37f96a5e46038e117853ac8637a9705fd
Closes-Bug: #1304105
(cherry picked from commit ec24d2cb3a)

etc/neutron/plugins/bigswitch/restproxy.ini

@@ -103,7 +103,7 @@ servers=localhost:8080
 # Default: ovs
 # virtual_switch_type = ovs
 
-[security_group]
+[securitygroup]
 # Controls if neutron security group is enabled or not.
 # It should be false when you use nova security group.
 # enable_security_group = True

etc/neutron/plugins/ml2/ml2_conf.ini

@@ -56,7 +56,7 @@
 # vxlan_group =
 # Example: vxlan_group = 239.1.1.1
 
-[security_group]
+[securitygroup]
 # Controls if neutron security group is enabled or not.
 # It should be false when you use nova security group.
 # enable_security_group = True