teuthology/suite: create nested matrix subsets

teuthology/suite: create nested matrix subsets

The general idea is to allow the `%` convolution operator to also subset
the resulting matrix. This is done by specifying a number of divisions
for the subset in the `%` file. Such as:

dir/%:

    8

This commit maps a matrix index range of `[0, Subset.size())` to the
matrix it is taking a subset of, `[0, Matrix.size())`. To get full
coverage, a random number is used to specify "which" subset to use.
Contrast with the `--subset` argument to `teuthology-suite` which lets
you specify which subset.

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