Handle proxy environment variables for epel-release install

Use yum_install() to support proxy environment variables when
epel-release package is installed from epel-bootstrap repo.

Change-Id: I45f7a38c6c71bbce07bd0d21c1ac8a75c04113cb
Closes-Bug: 1426221
diff --git a/stack.sh b/stack.sh
index 44a0743..58b4479 100755
--- a/stack.sh
+++ b/stack.sh
@@ -250,8 +250,10 @@
 enabled=0
 gpgcheck=0
 EOF
-    # bare yum call due to --enablerepo
-    sudo yum --enablerepo=epel-bootstrap -y install epel-release || \
+    # Enable a bootstrap repo.  It is removed after finishing
+    # the epel-release installation.
+    sudo yum-config-manager --enable epel-bootstrap
+    yum_install epel-release || \
         die $LINENO "Error installing EPEL repo, cannot continue"
     # epel rpm has installed it's version
     sudo rm -f /etc/yum.repos.d/epel-bootstrap.repo