Use bash as the default shell for command execution in crosperf.

subprocess.Popen defaults to /bin/sh, however, the intended behavior
is running them with bash.

BUG=None
TEST=Synatx error in . remote_access.sh is gone. Crosperf unit tests
passed.

Change-Id: I059ba9736e5ad3023b5c0a2201f6b40d11ddbe6e
Reviewed-on: https://chromium-review.googlesource.com/452862


Commit-Ready: Ting-Yuan Huang <laszio@chromium.org>
Tested-by: Ting-Yuan Huang <laszio@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>