bug fix: instantiate the Yum manager when update the yum packages list

an instance just refresh packagekit once.

for example:
$ sudo PYTHONPATH=src python -c '
> from yum import YumBase
> YumBase().doPackageLists("updates")
> _yb = YumBase()
> _yb.doPackageLists("updates")
> _yb.doPackageLists("updates")
> '
Loaded plugins: langpacks, refresh-packagekit
Loaded plugins: langpacks, refresh-packagekit

you can seen the second _yb.doPackageLists will not refresh packagekit.

The apt manager may also has the same problem.
But not test it on ubuntu, will send a patch later after I have check.

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