Make ssh key injection work with xenapi agent

If there's 'key_data' on an instance, and instance is not Windows, use
file injection to inject /root/.ssh/authorized_keys.  If a user
specifies their own /root/.ssh/authorized_keys via 'injected files',
that will take precedence.

Fixes bug 1152713

Note: Images for Xen that use nova-agent will need to have /root/.ssh
pre-created with modes 0700 on those directories.

DocImpact

Change-Id: I190c6b3f67099061b5316baa45b8bfcd1a5e53f6
Merge "Make ssh key injection work with xenapi agent"