[autoscaler] Make it possible to specify availability zone

[pcmoritz]

I was running into this on the current master, wanted to start a bunch of g2.2xlarges. According to

ray/python/ray/autoscaler/aws/config.py

Line 139 in d21ea0c

reverse=True, # sort from Z-A

the script used the availability zone that comes last in the alphabet (us-east-1f in my case) which didn't have these instances.

For now I'm working around it by changing it to reverse=False.

[richardliaw]

Another current workaround is specifying the subnet corresponding to the availability zone.

[robertnishihara]

Fixed by #1393