teuthology: add lua based fragment merge scripting

teuthology: add lua based fragment merge scripting

As part of this change, there is a new generator design for producing
job configs. YAML fragments are memoized and merged manually to avoid
expensive and unnecessary parsing of the merged fragments. This provides
for a dramatic speedup in processing matrices with large numbers of
jobs. For rados suite with --subset 1/1000, this branch is 5x faster
(77s vs. 15s). (Note: the difference shrinks when there are fewer or
jobs or larger subsets are used due to cycling and the matrix generation
dominating runtime.)

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>