don't fail on dirty logs with grenade

because grenade is upgrading from old to new we might actually
expect the logs to be dirtier than in upstream tempest. The grenade
logs weren't scrubbed in the same ways during the development here
as the tempest regular runs.

Change-Id: Id1bcc2cc85e73a414d382756a65ea1d80dc10b00
Closes-Bug: #1259907