teuthology-suite: add --seed option for repeatable random test

teuthology-suite: add --seed option for repeatable random test

currently --rerun does not match tests of
'supported-random-distro$/ubuntu_latest.yaml' with
'supported-random-distro$/centos_latest.yaml'. the former could be part
of description of a failed test, the latter is a a part of job
description generated by build_matrix(). because the '$' operator
instructs theuthology to choose a random file under the directory ending
with '$', and we expand the '$' to a randomly picked file *before*
filtering the generated job list with the filter collected from the
failed tests, there is good chance that the job descriptions of the
failed jobs in self.args.filter_in cannot match with the randomly
generated ones.

so, we introduce an argument '--seed' for teuthology-suite for the
repeatable random test. this argument allows user to specify a seed for
tne RNG used by build_matrix().

Signed-off-by: Kefu Chai <tchaikov@gmail.com>