StartShell.run to take this as None for non returning commands

[vnitinv]

For non returning calls (for example "monitor traffic interface fxp0") user can pass this as None for which the data will be calculated till timeout value.

    with StartShell(dev) as ss:
        print ss.run('cli -c "monitor traffic interface fxp0"', this=None, timeout=10)

[coveralls]

Coverage increased (+0.0007%) to 98.66% when pulling c68901c on vnitinv:start_shell_fix into 83891e0 on Juniper:master.