ec2tag watcher needs to back off if Client.RequestLimitExceeded is returned

[ianbamforth]

If you have a reasonable number of ec2tag watchers running, you can hit the rate limiter. Rather than a fixed check_interval, there should be a way of doing some kind of backoff (as per the AWS docs). If the Client.RequestLimitExceeded is returned, then there definitely needs to be a backoff - otherwise you just continue to hammer on the door, and the rate limit exceedance will never go away.

I'm yet to discover what the rate limit is, but I hit it today with ~6 servers each watching ~24 ec2tags (plus various other odds and sods running in the background) and the default check_interval

[ianbamforth]

Even better would be to somehow combine the requests made to EC2 in the background - if you have 100 watchers that all need to get the Tags every 15 seconds, that can be achieved with a single request - rather than 100 requests each with a filter applied.