add cgroups memory task

This is important if we want Nomad to regulate and limit the amount of 
memory that jobs use.

src/steps-preinstall/50-cgroups-memory.sh

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+
+check_cgroups_memory() {
+	task_begin "Checking memory cgroups";
+	
+	cgroups_enabled="$(awk '/memory/ { print $2 }' < /proc/cgroups)";
+	
+	if [[ "${cgroups_enabled}" -ne 0 ]]; then
+		echo ">>> memory cgroups already enabled";
+		return 0;
+	fi
+	
+	
+	filepath_cmdline="/boot/cmdline.txt";
+	if [[ ! -e "${filepath_cmdline}" ]]; then
+		filepath_cmdline="/boot/firmware/cmdline.txt";
+	fi
+	if [[ ! -e "${filepath_cmdline}" ]]; then
+		echo ">>> Failed to find cmdline.txt; can't check for cgroups";
+		return 1;
+	fi
+	
+	if grep -q cgroup_enable=memory /boot/cmdline.txt; then
+		echo ">>> memory cgroups already present in cmdline.txt, a reboot is required to apply the update";
+		return 0;
+	fi
+	
+	echo ">>> memory cgroups not present in cmdline.txt, enabling....";
+	(tr -d '\n' <"${filepath_cmdline}" && echo " cgroup_enable=memory cgroup_memory=1") | sudo tee "${filepath_cmdline}.new";
+	
+	sudo mv "${filepath_cmdline}" "${filepath_cmdline}.old-$(date +"%Y-%m-%d")";
+	sudo mv "${filepath_cmdline}.new" "${filepath_cmdline}";
+	
+	echo ">>> New contents of cmdline.txt:";
+	cat "${filepath_cmdline}";
+	echo ">>> A reboot is required to apply the changes.";
+	
+	task_end "$?" "Failed to check memory cgroups"
+}