Add DAS caching option to runTheMatrix.py

`das_client.py` has now an option to specify canned queries results in
case of DAS outages. This commit adds support for such a new option
to runTheMatrix.py so that jobs which need to query DAS simply
"pro-forma" (i.e. to make sure we do exactly what woul be done in
real production) are not stopped by occasional outages.

The goal is not to replace DAS queries, but simply to provide a
fallback option for the cases where we know data is there, most
notably IBs and pull request automated tests.

See also dmwm/DAS#4142 .