Remove unnecessary environment variables

[fabiob]

When running under docker-compose, the export -p command export a variable with an invalid name:

bash-4.3# export -p
declare -x DOCKERIZE_VERSION="0.4.0"
declare -x DOMAINS="..."
declare -x EMAIL="..."
declare -x HAPROXY_IMAGE="dockercloud/haproxy:1.6.6"
declare -x HOME="/root"
declare -x HOSTNAME="1d22be675798"
declare -x OLDPWD
declare -x PATH="/opt/letsencrypt/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
declare -x PWD="/"
declare -x SHLVL="1"
declare -x TERM="xterm"
declare -x affinity:container

The presence of this variable prevents the cronjob from running:

bash-4.3# /etc/periodic/daily/certbot 
/opt/letsencrypt/etc/global.env: line 11: declare: `affinity:container': not a valid identifier

This patch ignores this variable when writing to the global.env file.