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 export_proxy_variables
* Force an update with install_prereqs.sh -f or by setting
  FORCE_PREREQ=true

Fixed an issue with exit/return in tools/install_prereqs.sh

Change-Id: I9a62090ad2f900b9b150cacb9cb02b326cb46972
diff --git a/tools/install_prereqs.sh b/tools/install_prereqs.sh
new file mode 100755
index 0000000..4d151db
--- /dev/null
+++ b/tools/install_prereqs.sh
@@ -0,0 +1,82 @@
+#!/usr/bin/env bash
+
+# **install_prereqs.sh**
+
+# Install system package prerequisites
+#
+# install_prereqs.sh [-f]
+#
+# -f        Force an install run now
+
+if [[ -n "$1" &&  "$1" = "-f" ]]; then
+    FORCE_PREREQ=1
+fi
+
+# If TOP_DIR is set we're being sourced rather than running stand-alone
+# or in a sub-shell
+if [[ -z "$TOP_DIR" ]]; then
+    # Keep track of the devstack directory
+    TOP_DIR=$(cd $(dirname "$0")/.. && pwd)
+
+    # Import common functions
+    source $TOP_DIR/functions
+
+    # Determine what system we are running on.  This provides ``os_VENDOR``,
+    # ``os_RELEASE``, ``os_UPDATE``, ``os_PACKAGE``, ``os_CODENAME``
+    # and ``DISTRO``
+    GetDistro
+
+    # Needed to get ``ENABLED_SERVICES``
+    source $TOP_DIR/stackrc
+
+    # Prereq dirs are here
+    FILES=$TOP_DIR/files
+fi
+
+# Minimum wait time
+PREREQ_RERUN_MARKER=${PREREQ_RERUN_MARKER:-$TOP_DIR/.prereqs}
+PREREQ_RERUN_HOURS=${PREREQ_RERUN_HOURS:-2}
+PREREQ_RERUN_SECONDS=$((60*60*$PREREQ_RERUN_HOURS))
+
+NOW=$(date "+%s")
+LAST_RUN=$(head -1 $PREREQ_RERUN_MARKER 2>/dev/null || echo "0")
+DELTA=$(($NOW - $LAST_RUN))
+if [[ $DELTA -lt $PREREQ_RERUN_SECONDS && -z "$FORCE_PREREQ" ]]; then
+    echo "Re-run time has not expired ($(($PREREQ_RERUN_SECONDS - $DELTA)) seconds remaining); exiting..."
+    return 0
+fi
+
+# Make sure the proxy config is visible to sub-processes
+export_proxy_variables
+
+
+# Install Packages
+# ================
+
+# Install package requirements
+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 $FILES/rpms-suse)
+else
+    exit_distro_not_supported "list of packages"
+fi
+
+if [[ -n "$SYSLOG" && "$SYSLOG" != "False" ]]; then
+    if is_ubuntu || is_fedora; then
+        install_package rsyslog-relp
+    elif is_suse; then
+        install_package rsyslog-module-relp
+    else
+        exit_distro_not_supported "rsyslog-relp installation"
+    fi
+fi
+
+
+# Mark end of run
+# ---------------
+
+date "+%s" >$PREREQ_RERUN_MARKER
+date >>$PREREQ_RERUN_MARKER