Add combinations file to make entire rapid suite run with both
varlen and dictionary encoding.

If tests have to be run with the default encoding, run MTR
with --skip-combinations option ( *_dict and *_varlen tests always run).
This will be added for the per-push runs too.

If tests have to be run with both varlen and dictionary
encodings, remove the --skip-combinations option.
The daily runs will run with the combinations to ensure
more coverage is attained.

Change-Id: Id08e1ada70d16b340cc2c0c77ee8f84718858afb