CloudWatch autoscaling based on ELB latency

By default, sets up CloudWatch alarms that scale each fleet in
response to the Latency metric on their respective ELBs.  Also includes
some defaults in mediawiki.yaml to prevent overzealous autoscaling from
knocking tiers down to a single instance.
Argh, typo fix
Merge pull request #68 from huit/feature/cloudwatch_monitors

CloudWatch autoscaling based on ELB latency