Fix pip distro package removal for focal
Fix pip distro package removal for focal

Ubuntu Focal doesn't have python-pip, only python3-pip. Trying to
uninstall a package that apt doesn't know about (installed or
uninstalled) results in a nonzero exit code so devstack fails. This
patch makes the package removal safer for both python2 and python3 cases
by checking first if the package exists.

Change-Id: I3b1118888cb0617ffb99b72c7e9a32308033783e

nawawi/openstack,nawawi/openstack,openstack-dev/devstack,openstack-dev/devstack