bug fix: Instantiate the apt-get manager when update the apt-get packages list

It is needed to always get the correct information from apt-get.
Otherwise, even after updating packages the same packages will be listed
to be updated as they are kept in the apt-get instance.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
Signed-off-by: Aline Manera <alinefm@br.ibm.com>