Fix misleading error message about failed dhcp notifications

This is achieved by adjusting the log traces and ensuring
that the right log errors are emitted based on the status
of the network.

Also, the patch drastically simplifies the structure of the
notification method and unit tests to increase coverage.

Closes-bug: #1289130

Change-Id: I7cc78bba81c516380fc93a68aa7b295312a88e29
LBaaS: small cleanup in agent device driver interface

remove unnecessary 'context' parameter

Change-Id: Iaa3896f89760817af9d72e653252830f8a33e390
Closes-Bug: #1294096
Merge "Fix misleading error message about failed dhcp notifications"
Merge "LBaaS: small cleanup in agent device driver interface"