Fix handling of pip and virtualenv on openSUSE

openSUSE's python-virtualenv rpm depends on the python-pip rpm, but
tools/install_pip.sh prefers to deinstall the latter (if installed) and
install pip directly from upstream source instead.

This deinstallation of python-pip will break if attempted via rpm -e,
since rpm does not transitively remove dependents (in this case
python-virtualenv).  In contrast, "zypper rm" does, so we switch to
that.

It is safe to remove the python-virtualenv package, since stack.sh will
install virtualenv via pip instead.

Change-Id: I5bc23de0f2de2e3940c4be3b76b7c0634836239b
Merge "Fix handling of pip and virtualenv on openSUSE"