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