Create tools/install_prereqs.sh

* Factor system package prereq installs out to tools/install_prereqs.sh
* Set minimum time between runs with PREREQ_RERUN_HOURS
  default = 2 hours
* Create re_export_proxy_variables

Change-Id: I4a182b1da685f403d6abdd8540d2114796c01682
diff --git a/stack.sh b/stack.sh
index da0faed..c8b8db4 100755
--- a/stack.sh
+++ b/stack.sh
@@ -644,25 +644,7 @@
 
 # Install package requirements
 echo_summary "Installing package prerequisites"
-if is_ubuntu; then
-    install_package $(get_packages $FILES/apts)
-elif is_fedora; then
-    install_package $(get_packages $FILES/rpms)
-elif is_suse; then
-    install_package $(get_packages $FILES/rpms-suse)
-else
-    exit_distro_not_supported "list of packages"
-fi
-
-if [[ $SYSLOG != "False" ]]; then
-    if is_ubuntu || is_fedora; then
-        install_package rsyslog-relp
-    elif is_suse; then
-        install_package rsyslog-module-relp
-    else
-        exit_distro_not_supported "rsyslog-relp installation"
-    fi
-fi
+$TOP_DIR/tools/install_prereqs.sh
 
 if is_service_enabled rabbit; then
     # Install rabbitmq-server