Mock seamicroclient lib in unit tests if not present

Mock the python-seamicroclient library from within ironic/tests/drivers
so that test_seamicro.py can import ironic/drivers/modules/seamicro.py
and run the unit tests on it, even when the external library is not
present.

Change-Id: I4f2cd75b2c64a5a280a5d45d5cea058fd432815b
Closes-bug: #1312321
Merge "Mock seamicroclient lib in unit tests if not present"