DHCP agent should exit if the interface driver isn't provided

[Édouard Thuleau]

If the configuration flag 'interface_driver' isn't set in the configuration file of DHCP agent, the agent should exit with error log just after trying to start.
Like it's done with l3 agent: https://github.com/openstack/quantum/blob/master/quantum/agent/l3_agent.py#L158

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/22290

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/22290
Committed: http://github.com/openstack/quantum/commit/61b2cf8e6e7cf520876443d1d64c100645746ae4
Submitter: Jenkins
Branch: master

commit 61b2cf8e6e7cf520876443d1d64c100645746ae4
Author: Gary Kotton <email address hidden>
Date: Tue Feb 19 11:34:00 2013 +0000

    Exit if DHCP agent interface_driver is not defined

    Fixes bug 1130052

    In addition to this:
    Uses "raise SystemExit()" instaed of the exit for the L3
    agent.

    Change-Id: I8130745c9de62619189e158944fa9e4ed25be774