Ubuntu Packaging: add python-apt to dependency

In ovirt-guest-agent/GuestAgentLinux2.py we try to import apt module.
This module is provided by python-apt on Debian and Ubuntu. This patch
adds python-apt to package dependency.

Change-Id: I7a248b43c541ded03a26625896881d89a0d6401d
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>