Make variants sticky when implemented

[pearce8]

I removed the use of sticky variant in Tioga scheduler in #435 because sticky variants are not currently implemented. We need to go through all variants and decide which should be sticky once that is an option.

@becker33 do we need a change in Ramble for this?

File "/home/runner/work/benchpark/benchpark/lib/benchpark/directives.py", line 140, in variant
    raise NotImplementedError("Sticky variants are not yet implemented in Ramble")