Add "only" selector to the list filter.

This enables decorators as
@harness.archtest(only = ["64"])
@harness.platformtest(only = ["linux"])
@harness.distrotest(only = ["ubuntu"])

Which should be of obvious use. Unit tests for the list filtering util.

Change-Id: I7eef8e211eb7510e4ccc0382479679551387c230