4670: Add a post-create hook to Node Manager for EC2 tagging.

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.