Fix greenplum_path script for OSS Ubuntu

We do not vendor python on OSS Ubuntu 16.04 and therefore
`$GPHOME/ext/python` doesn't exist. This causes the system python to
fail with an error message about not being able to find the site module.

This commit updates the logic in generate-greenplum_path.sh to check if
we are vendoring python and if we are, it includes setting `$PYTHONHOME`
in greenplum_path.sh. If we are vendoring python, then at build-time
`$PYTHONHOME` is set to point at our vendored python.

[#173046174]
Authored-by: NBradford D. Boyle <bradfordb@vmware.com>

gpMgmt/bin/generate-greenplum-path.sh

@@ -9,10 +9,22 @@ GPHOME_PATH="$1"
 cat <<EOF
 GPHOME="${GPHOME_PATH}"
 
-PYTHONHOME="${GPHOME}/ext/python"
+EOF
+
+if [ -x "${PYTHONHOME}/bin/python" ]; then
+	cat <<-"EOF"
+	PYTHONHOME="${GPHOME}/ext/python"
+	export PYTHONHOME
+
+	PATH="${PYTHONHOME}/bin:${PATH}"
+	LD_LIBRARY_PATH="${PYTHONHOME}/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
+	EOF
+fi
+
+cat <<"EOF"
 PYTHONPATH="${GPHOME}/lib/python"
-PATH="${GPHOME}/bin:${PYTHONHOME}/bin:${PATH}"
-LD_LIBRARY_PATH="${GPHOME}/lib:${PYTHONHOME}/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
+PATH="${GPHOME}/bin:${PATH}"
+LD_LIBRARY_PATH="${GPHOME}/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
 
 if [ -e "${GPHOME}/etc/openssl.cnf" ]; then
 	OPENSSL_CONF="${GPHOME}/etc/openssl.cnf"
@@ -20,7 +32,6 @@ fi
 
 export GPHOME
 export PATH
-export PYTHONHOME
 export PYTHONPATH
 export LD_LIBRARY_PATH
 export OPENSSL_CONF