Clean up bindep invocation in cache-bindep element

Invoke bindep from the bindir for its virtualenv in /usr/bindep-env,
and guard its exit code with true since its normal behavior is to
return zero only when all needed packages are already present.

While here, replace calls to tempfile with mktemp since the former
is not readily available on CentOS 6.x.

Change-Id: I926e73f305b1b54f6855cc4acf3407e91b88213a

nodepool/elements/cache-bindep/install.d/50-cache-pkgs

@@ -21,10 +21,10 @@ FALLBACKFILE=jenkins/data/bindep-fallback.txt
 PACKAGES=`git --git-dir=$FALLBACKREPO/.git show master:$FALLBACKFILE`
 REQSFILE=other-requirements.txt
 for REPO in /opt/git/*/* ; do
-    BRANCHES=`git --git-dir=$REPO/.git branch -r|grep '^  origin/[^H]'`
+    BRANCHES=`git --git-dir=$REPO/.git branch -r | grep '^  origin/[^H]'`
     for BRANCH in $BRANCHES ; do
         REQS=`git --git-dir=$REPO/.git show $BRANCH:$REQSFILE 2>/dev/null \
-            ||true`
+            || true`
         if [ -n "$REQS" ] ; then
             PACKAGES=`echo -e "$PACKAGES\n$REQS" | sort -u`
         fi
@@ -32,10 +32,10 @@ for REPO in /opt/git/*/* ; do
 done
 # TODO(fungi) once https://review.openstack.org/195201 appears in a bindep
 # release, replace the next four lines with:
-# PACKAGELIST=`echo "$PACKAGES" | bindep -b -f -`
-PACKAGESFILE=`tempfile`
+# PACKAGELIST=`echo "$PACKAGES" | /usr/bindep-env/bin/bindep -b -f - || true`
+PACKAGESFILE=`mktemp`
 echo "$PACKAGES" > $PACKAGESFILE
-PACKAGELIST=`bindep -b -f $PACKAGESFILE`
+PACKAGELIST=`/usr/bindep-env/bin/bindep -b -f $PACKAGESFILE || true`
 rm $PACKAGESFILE
 
 if grep "^CentOS release 6" /etc/redhat-release ; then