oeqa/selftest/case.py: Remove machine selection logic

The machine selection is an operation that needs to be made
in every test run, the best place to it is on the context
module.

Use self.tc.custommachine variable instead of use environment.

SIgned-off-by: Leonardo Sandoval <leonardo.sandoval.gonzalez@linux.intel.com>
Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>