4670: Node Manager handles more libcloud exceptions.

libcloud compute drivers (at least EC2 and GCE) raise bare Exceptions
when there's some problem talking to the cloud service.  The previous
code was expecting to see a LibcloudError, so it wouldn't handle these
errors as intended.

I didn't want to just catch errors with "except Exception" everywhere,
so I added an is_cloud_exception class method to our driver classes to
more accurately identify exceptions that represent trouble talking to
the cloud service.  It recognizes exact Exceptions, plus the other
classes we were catching before.

While I was at this, I gave more specific names to the wrapper methods
in compute node actor decorators, as a debugging aid.
4670: Add a post-create hook to Node Manager for EC2 tagging.

The previous code was relying on the post-create tagging in libcloud's
EC2 driver.  Unfortunately, that's not working out too well for us: if
it fails, you get no indication of that, and it doesn't get retried.
This moves the work up into Node Manager, where failures can be logged
and retried appropriately.

The retry support may be sufficient to resolve #4670.  If it's not,
then the additional logging will help us track down the root cause.
Merge branch '4670-node-manager-robust-tags-wip'

Closes #4670, #4812.