Fix ML2 securitygroup API enable/disable

Securitygroup API is enabled by default, and a dummy
firewall_driver is no longer needed. This patch
rearranges content to make it clear which files are
edited on which servers, and fixes the use of the flag
values.

Change-Id: I7b2a6e646bbf02aa79b39f7f118a2ce0f6bb6a40
Closes-Bug: 1332104
Merge "Fix ML2 securitygroup API enable/disable"