container_execute does not allow for interactive use

[Adrian Otto]

 In magnum/conductor/handlers/docker_conductor.py we define container_execute(), but we do not allow for the options to be set to allow a pseudo tty to be allocated to cause interactive operation with the command on the terminal.

Reference: http://docker-py.readthedocs.org/en/1.2.2/api/#exec_create

We pass the last argument as False, which disables the pseudo tty.

Correct this by allowing a -it command to "magnum container-exec" (consistent with docker's flags to request an interactive terminal) that causes that False value to be True.

Note that the CLI command may currently be "magnum container-execute", but changing that to "magnum container-exec" will be addressed in a separate bug.

[Adrian Otto (aotto)]

See Also: https://bugs.launchpad.net/magnum/+bug/1459877

[Chuenlye (chuenlye-leo)]

I picked up this bug to fix for OpenStack Upstream Training in Tokyo.
First attempt at contributing code. Please help in anyway you see fit.