Faststart should check /var vs. /home FS size and warn when /home is largest

[mbacchi]

Eucalyptus defaults to /var/lib/eucalyptus/instances for the instance directory, but on a vanilla CentOS/Redhat installation the /home filesystem is created much larger than /var. We should look at these sizes and warn when they don't have much capacity in /var/lib/eucalyptus/instances, compared to /home. We won't change anything related to the user's filesystems, just recommend that they don't have enough capacity for many instances and suggest that a directory in /home be used for the INSTANCE_PATH in the eucalyptus.conf file.

This is a common reason why users can't run more than a couple t2.micro instances upon completion of their faststart installation.