Provide diagnostics when task rate limiting.
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

seandst/nodepool,seandst/nodepool,Tesora/tesora-nodepool,Tesora/tesora-nodepool,citrix-openstack/nodepool,citrix-openstack/nodepool