doc: clarify requirement of obfsvpn endpoint port for openvpn remote
this also introduces OBFS4_ENDPOINT_PORT and renames OBFS4_ENDPOINT to OBFS4_ENDPOINT_PORT and updates the script 'run-openvpn.sh' to use these
this also introduces OBFS4_ENDPOINT_PORT and renames OBFS4_ENDPOINT to OBFS4_ENDPOINT_PORT and updates the script 'run-openvpn.sh' to use these