DHCP agent scheduler support for BigSwitch plugin

Adds support for the Neutron DHCP agent scheduler to the BigSwitch
plugin to support multiple neutron DHCP agents.

Implements: blueprint bsn-dhcp-agent-scheduler
Change-Id: I629ce73c0b97c2409b37afb85639282a116560b7
Remove database section from plugin.ini

Change https://review.openstack.org/#/c/34515 moved the database
config items to neutron.conf but left some commented out
[database] items. Remove them as well.

Closes-Bug: #1247589
Change-Id: I05913d0c27a683cec30cae31a1978612aa1e1360
Remove confusing comment and code for LBaaS

The comment "the driver is optional" is inconsistent with what actually
done. And "vif_driver = None" after raising an exception makes no sense.
So this patch removes these lines.

Change-Id: I7f3d5b6df61037307e952a478f47c59485601847
Merge "Remove confusing comment and code for LBaaS"
Updated from global requirements

Change-Id: I36e3ce085af0b1e9a7dfd91141789fdfbc2049a8
Merge "DHCP agent scheduler support for BigSwitch plugin"
Merge "Updated from global requirements"
Merge "Remove database section from plugin.ini"