Fix: Skip rescheduling networks if no DHCP agents available

Fix: Skip rescheduling networks if no DHCP agents available

This fixes commit 1318437a0caf38e695a819848832a955fef7d909
which has inaccurate check for dead agents: in a case where
there is only one network scheduled to a subset of dhcp agents,
number of all dhcp agents will be not equal to the number of
dead binded agents

The unit test is updated to cover the described case.

Closes-Bug: #1461714
Change-Id: I1c9501316c931293aa8ba755a98779a7da27578d