Don't disable SELinux in cloud-init's boothook.sh

This is a first step towards fixing #1257410 as outlined in the bug
report.

Disabling SELinux is not necessary, but the fact that we're using both
the `user` directive in cloudinit/config and `useradd` in boothook.sh
is a bit confusing so this documents the reasons for both.

Change-Id: Ife4fec99650341de698eb51741d96f95de00dcf0
Partial-Bug: #1257410

heat/cloudinit/boothook.sh

@@ -1,5 +1,9 @@
 #!/bin/bash
-command -v setenforce >/dev/null 2>&1 && setenforce 0
+
+# FIXME(shadower) The `useradd` and `sudoers` lines are a workaround for
+# cloud-init 0.6.3 present in Ubuntu 12.04 LTS:
+# https://bugs.launchpad.net/heat/+bug/1257410
+# Once we drop support for it, we can safely remove them.
 useradd -m @INSTANCE_USER@
 echo -e '@INSTANCE_USER@\tALL=(ALL)\tNOPASSWD: ALL' >> /etc/sudoers
 

heat/cloudinit/config

@@ -1,3 +1,7 @@
+# Set the SSH key provided by Nova to this user.
+# On cloud-init 0.7.x (anything except Ubuntu 12.04 LTS which ships 0.6.3) this
+# also creates the user and sets up passwordless sudo if the user isn't present
+# already.
 user: @INSTANCE_USER@
 
 # Capture all subprocess output into a logfile