Make key injection destination configurable

[Garrett Holmstrom]

The typical EC2 image contains an init script that downloads SSH keys to whichever place on the instance's filesystem makes sense. This also works in Eucalyptus's managed mode. Eucalyptus, however, overwrites /root/.ssh/authorized_keys when a new instance starts whenever the NC is capable of mounting the root filesystem. While useful, this key injection makes a few assumptions:

* Images are always configured to allow root to log in via SSH
* Images never contain SSH keys in /root/.ssh/authorized_keys
* In managed mode, users always want Eucalyptus to inject keys instead of, or in addition to, downloading them via scripts baked into images as they do on EC2

Making the location that the SSH keys go configurable should make it possible to log in as a non-root user via the key-injection mechanism. In addition, it would provide an easy way to skip key injection altogether should an image not require it.

[Andy Grimm (agrimm)]

This issue is now being tracked upstream at http://eucalyptus.atlassian.net/browse/EUCA-2772

Please watch that issue for further updates.