Support exclusion rules in combinations

Support exclusion rules in combinations

Like py.test we need to be able to mark certain combination
elements with exclusion rules.   Add additional logic
to pytestlplugin and exclusions so that the exclusion decorators
can be added to the combination tuples, where they will be applied
to the decorated function along with a qualifier that the test
arguments need to match what's given.

Change-Id: I15d2839954d77a252bab5aaf6e3fd9f388c99dd5