Tighten exception handler for import_object

Tighten exception handler for import_object

oslo_utils raise ImportError if import fails. We should propagate other
failures to callers. Otherwise we may hide issues.

Also report exact failure from import_object in case L3 agent fails to
import interface_driver.

As part of the job, consolidated code to load interface driver into
common function.

Also, stopped checking for specific log messages in dhcp and l3 agent
unit tests: it's too fragile and actually not something we need a unit
test for.

Not to introduce more work for people who handle py3 porting effort,
added the unit test into the list of those that are executed for py34
job until the whole suite is ready for python3.

Change-Id: I10cdb8414c9fb4ad5cfd3f3b2630811f50ffb0c7