Add /exec/{id}/info support

[christopherobin]

Adds a new boolean parameter exec_id to execute to return the execution id as a second value.
That value can then be fed to execute_inspect to get informations about the execution (running status, exit code, etc...).

Usage:

out, exec_id = c.execute(container, ['ls', '/foobar'], exec_id=True)
info = c.execute_inspect(exec_id)
if info.get('ExitCode') == 2:
    raise IOError('Folder /foobar does not exists')

[MiLk]

👍

[shin-]

Thanks! I actually took a different approach in #568, but end result is essentially the same.