Make long polling interval a command line parameter option

[jhorman]

Also lowers the default polling interval to 10, 20 is just a long time to wait, and leads to shutdowns taking a long time.

Fixes #60

[andrewgross]

Hey jhornman,

Thanks for submitting this as well. Looks like a solid addition. The tests are failing due to an issue with boto. We can figure out how to migrate the tests to boto3 and avoid the problem, or fix the config so it isn't looking at GCE.

I can work on adding this in shortly. Thanks again!