software_manager: add support for single package upgrade

Add optional package name arg so we can upgrade individual
packages. This was made in all known backends so to maintain
a uniform interface.

Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #764 from autotest/software-manager

software_manager: add support for single package upgrade