Document neutron-ovs-cleanup

Adds some documentation about the need to run neutron-ovs-cleanup
to ensure that DHCP agent and L3 agent perform properly on reboot.

Closes-Bug: 1156861

Change-Id: I556a15bd2aed18513a57eced465476181097e01d
backport: havana
Merge "Document neutron-ovs-cleanup"