Unify and document environment variables for debugging
There are a lot of environment variables used for debugging:
DEBUG
LEAP_DRYRUN
LEAP_PRIVATE_BRIDGE_CERT
LEAP_KCP
OPENVPN_VERBOSITY
SKIP_VERSION_CHECK
LEAP_PROVIDER
LEAP_PRIVATE_BRIDGE_CERT
MOTD_URL
LEAP_OPENVPN_EXTRA_CONFIG
LEAP_GW
UDP
They are not documentend in the README. Maybe we can remove some of them. Maybe combine with (Unify logging and add log level) #771 (closed)