Provide diagnostics when task rate limiting.

Rate limiting is necessary for some clouds, but it can be helpful to
be able to see when it is kicking in. We expect it to kick in when
making API requests from public cloud providers with ratelimits (all
our current ones).

Change-Id: Ieca7b49f1719ac4b4a326ae6d3682f1bca08ad54
Default to a ratelimit of 2/second for API calls

We make a lot of API calls when deleting servers - 8 in fact to
enumerate and delete floating ips, keypairs and the server. A 1 second
rate limit between calls makes each delete take 8 seconds to move
past, and then if we're deleting 50 servers on a single provider we
won't get through submitting the deletes for all 50 for 500 seconds,
or 8 minutes.

Change-Id: I4ce86c38030e63db3547043134940fb9cf01eafa