cli integration: allow driver selection via vars

This makes it possible to choose the graphdriver and the execdriver
which is going to be used for the cli integration tests.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
cli integration: fix wait race

The wait at the end of cli integration script could end up failing if
the process had already exited. This was making it look like the tests
have failed.

This change fixes the problem.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #5027 from unclejack/cli_integration_add_docker_options

Cli integration: add docker options & fix the wait at the end