Allow users to specify the maximum bid price for spot instances

https://github.com/AppScale/appscale-tools/pull/180 enables users to use spot instances when running in EC2. In that pull, we automatically choose the maximum bid price based on the average bid price in the past. This pull goes one step further and allows users to manually specify what they want the max bid price to be, in lieu of using our algorithm.

As this pull branches off of (https://github.com/AppScale/appscale-tools/pull/180), don't merge this branch in until that branch is merged.