e2e_node: use mktemp when building nsenter on trusty

There's a bit of a hack in place to insure nsenter is present on Ubuntu
trusty, which doesn't otherwise include it.  This downloads util-linux
to a hard coded directory in /tmp which is a bad practice.  Even though
"this is just a test case" it should properly use mktemp.

Signed-off-by: Tim Pepper <tpepper@vmware.com>

test/e2e_node/environment/setup_host.sh

@@ -57,8 +57,8 @@ cat /etc/*-release | grep "ID=ubuntu"
 if [ $? -eq 0 ]; then
   if ! which nsenter > /dev/null; then
      echo "Do not find nsenter. Install it."
-     mkdir -p /tmp/nsenter-install
-     cd /tmp/nsenter-install
+     NSENTER_BUILD_DIR=$(mktemp -d /tmp/nsenter-build-XXXXXX)
+     cd $NSENTER_BUILD_DIR
      curl https://www.kernel.org/pub/linux/utils/util-linux/v2.24/util-linux-2.24.tar.gz | tar -zxf-
      sudo apt-get update
      sudo apt-get --yes install make
@@ -67,7 +67,7 @@ if [ $? -eq 0 ]; then
      ./configure --without-ncurses
      make nsenter
      sudo cp nsenter /usr/local/bin
-     rm -rf /tmp/nsenter-install
+     rm -rf $NSENTER_BUILD_DIR
    fi
 fi