dockertest.environment: Add EventHandler class for handling events

This class parses the system logs and helps to wait_for given list
of events.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
subtests.docker_cli.start: Modify short_term_app to use events

This patch uses custom EventDockerCmd to execute command and start
the timeout from the time container was actually started. This should
fix the problem with downloading images before the test.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>