nodepool/elements : use abstracted commands

On platforms such as CentOS 8 which are python3 first, "virtualenv"
and "pip" may not exist (removed to avoid any confusion over them
being v2 or v3 commands).

The referenced dib change exports new variables that should be correct
on all platforms for creating virtualenv's and pip installs.

(note will require DIB release to be active on builders)

Depends-On: https://review.opendev.org/684462
Change-Id: I3414fb9e503f94ff744b560eff9ec0f4afdbb50e

nodepool/elements/infra-package-needs/install.d/10-pip-packages

@@ -25,6 +25,6 @@ packages='tox'
 # non-system root installed packages can interfere with things
 if [ "${DISTRO_NAME}" != "gentoo" ]; then
     for package in $packages ; do
-        pip install $package
+        $DIB_PYTHON_PIP install $package
     done
 fi

nodepool/elements/infra-package-needs/install.d/40-install-bindep

@@ -22,5 +22,5 @@ if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
 fi
 set -e
 
-virtualenv /usr/bindep-env
+$DIB_PYTHON_VIRTUALENV /usr/bindep-env
 /usr/bindep-env/bin/pip install bindep

nodepool/elements/nodepool-base/install.d/91-venv-os-testr

@@ -18,5 +18,5 @@ if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
 fi
 set -e
 
-virtualenv /usr/os-testr-env
+$DIB_PYTHON_VIRTUALENV /usr/os-testr-env
 /usr/os-testr-env/bin/pip install os-testr