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 ec24d2cb3ac17a86ee5c4626c625131c10732143)