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

Author: cmurphy

tools/install_pip.sh

@@ -133,8 +133,12 @@ get_versions
 # results in a nonfunctional system. pip on fedora installs to /usr so pip
 # can safely override the system pip for all versions of fedora
 if ! is_fedora  && ! is_suse; then
-    uninstall_package python-pip
-    uninstall_package python3-pip
+    if is_package_installed python-pip ; then
+        uninstall_package python-pip
+    fi
+    if is_package_installed python3-pip ; then
+        uninstall_package python3-pip
+    fi
 fi
 
 install_get_pip