Bug Fix - Correctly installing cobbler 2.6.3 on Upgrade

This check-in:
- Modifies Ubuntu Installer script to correctly upgrade cobbler to 2.6.3 on upgrade of Smgr.
- Changed method to completely uninstall contrail-server-manager to prevent broken cache due to dependency issues.

Change-Id: I5964cfdc10319265c89a68e2f60b2d3e827791de