Don't expand shell variable in the custom_user_script

The provided custom script is copied into the body of the user_data.sh
script run by the build_ubuntu_ami before being written out to
disk. This causes any shell variables to be expanded in the parent
script.

For example, consider the following custom script:

    . /etc/lsb-release
    REPO_DEB_URL="http://apt.puppetlabs.com/puppetlabs-release-${DISTRIB_CODENAME}.deb"
    #...
    repo_deb_path=$(mktemp)
    wget --output-document=${repo_deb_path} ${REPO_DEB_URL} 2>/dev/null
    dpkg -i ${repo_deb_path} >/dev/null

This gets exanded into the following broken code:

    . /etc/lsb-release
    REPO_DEB_URL="http://apt.puppetlabs.com/puppetlabs-release-.deb"
    #...
    repo_deb_path=/tmp/tmp.lmtBQ780ba
    wget --output-document=  "http://apt.puppetlabs.com/puppetlabs-release-.deb" 2>/dev/null
    dpkg -i >/dev/null

There are two problems here:

1. ${repo_deb_path} ends up empty in the final script.

2. /etc/lsb-release is not sourced in the parent script, so
  ${DISTRIB_CODENAME} is expanded to an empty string.

Quoting the custom script within the body of the user_data.sh script
prevents variable expansion until the custom script is executed,
fixing the problem.