Conductor logs better error if seamicroclient missing

If the driver is installed and the python entrypoint discoverable, but
the external library is not present, the conductor will log exceptions
during startup and remove the *_seamicro drivers from the list of
drivers it advertises support for.

This patch tests the imports and raises one DriverNotFound error for
each affected driver if python-seamicroclient is not installed, instead
of raising several ImportErrors.

This also removes python-seamicroclient from requirements.txt

Change-Id: I5cde3c3e8c7cbc2bf4991c0ed390ac655dcdbc7c
Merge "Conductor logs better error if seamicroclient missing"