[FLOC-1091] Use a max_spot_price defined in the config file.

I'm not sure if this is a good idea and I haven't gone so far as to test it.

Fixes https://clusterhq.atlassian.net/browse/FLOC-1091
Choosing a sensible max spot price can be based off the following estimations:
* How many dollars we would rather spend than have Buildbot down
* How likely is it for a particular max price Buildbot will go down
* How much would it cost, do we think, to run Buildbot if we set a particular max price

Two of these can be aided with a comment about past performance (e.g. we had a spot price of 1.6, it went down after X months and took 2 hours to fix. We spent on average $N / month at that spot price). If it turns out that N is minuscule, let's not worry about this and just put a very high figure there. If N is relatively close to "how many dollars we would rather spend than have Buildbot down" then we should put thought into this.