Added Ubuntu support

This plugin is similar to RedHatOS plugin. This commit also fixes a
import error in CacheManager. The StackEnvironment was also updated
to be in sync with the direct booting functionality in Nova.
Fixed typo in code path for installing Ubuntu from network.
Added proper exit status for timed out installation.
Fixed inactivity check.
Merge pull request #10 from redhat-imaging/exit_codes

Fix inactivity timeout check