Add option to exec to set maximum procs (concurrent steps)

[raphendyr]

The exec command doesn't respect environment variable DRONE_RUNNER_MAX_RPOCS. I think it would be better. However, I implemented this single flag for the configuration parameter, which was the only one without a command line argument.

[tphoney]

What was the situation that this was important ?

[raphendyr]

I have around 50 parallel steps, which (trivially) resulted Drone for mac to run out of memory. I'm using drone-runner-docker exec in place of drone exec as the runner has better set of features. Additionally, if the latter is expect to be replaced by the former, I felt that it's better to contribute here.

Any case, the exec should also respect the environment variable DRONE_RUNNER_MAX_RPOCS, which would be better. That was just way bigger change, so I didn't want to look into that.

Note that the limitation is implemented, the exec subcommand just didn't set the value for it, i.e., the code used the unchanged default from the struct.

[RopoMen]

@tphoney what is status for this?