functions.guest: add iniset_sudo, a root wrapper around iniset

This changeset introduces iniset_sudo, a root wrapper around the
devstack function, iniset.

The wrapper function makes it easier to have comments explaining
lines changes to configuration files.

Instead of sourcing the devstack library in the root environment as
well, iniset_sudo works on a temporary copy of the configuration file
and uses root privileges only to copy the result back into place. It
would have to be modified before it could edit files that the script
user has no permission to read.

The changeset illustrates the use of the function with a patched
apt_install_mysql.sh

Change-Id: I416c0d14280e774a939d7bebaf7d45c3a488e763