Switch from try/finally to contextlib.ExitStack
Switch from try/finally to contextlib.ExitStack

CLOUDBLD-8136

There will be more cleanup to do after the remote host integration is
added. This is more convenient. And it's way cooler :)

Signed-off-by: Adam Cmiel <1217f9865bd733d1bcad0d0d64be310272c5592c@redhat.com>

projectatomic/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor,fr34k8/atomic-reactor