Use name for AMI/template limits.

This changes AMI-specific limits to use the name (description) instead of
the AMI to apply limits for templates. We accomplish this by setting a tag,
jenkins:description, on instances in order to identify what template they
belong to. Please notice that the jenkins: prefix is now reserved for internal
use. Do not set tags beginning with jenkins: in the configuration screen.
Merge branch 'description-instead-of-ami-for-limits' into force-mode