Handling of revision number and a subset of met. sites

[ccarouge]

This adds 2 things:

• better handling of the revision number per branch. We want to be able to run for a specified revision number or from the HEAD of each branch independently. Now each run of benchcab writes out the revision numbers used in a different file.
• run for a subset of the PLUMBER2 sites. It's now possible to specify sites to run in the config.yaml file under a met_subset list. For the moment, one needs to specify the exact filenames, it will need to be changed to specifying the site name instead and the code figuring out the filename for each site.