Add unit tests for dhcp scheduler module

[Eugene Nikanorov]

need to add UTs for dhcp agent scheduler module

[Armando Migliaccio (armando-migliaccio)]

https://review.openstack.org/#/c/74962/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/74962
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=237746a6d63c96726e7966770a602b62205ee333
Submitter: Jenkins
Branch: master

commit 237746a6d63c96726e7966770a602b62205ee333
Author: armando-migliaccio <email address hidden>
Date: Thu Feb 20 00:21:19 2014 -0800

    Fix DetachedInstanceError for Agent instance

    The stacktrace observed seems to hint to a
    situation where a session is not properly
    rolled back when duplicate agent entries are
    found.

    Since it is ok to deal with duplicates, the
    addition of agents to the DB must be done
    one by one, otherwise we go against the
    ATOMICITY property of a transaction of all
    or none. It's either that or I am barking at
    the wrong tree.

    This patch also added UT to cover affected
    code.

    Closes-bug: 1282421
    Related-bug: 1282253

    Change-Id: Ic5f77808cc10f885d67f5a9cf3b3836283692850