When Pungi receives a signal to terminate, it can sometimes get stuck
if there are threads running. It has to wait for all worker threads to
finish. They generally do finish, unless they get stuck waiting on a
subprocess.
This patch should reduce the likelihood of this happening by stopping
all subprocesses.
JIRA: RHELCMP-3056
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>