client should recognize NDSSERVER environment variable

This is port of trac issue #272

The client still fails to recognize the NDSSERVER environment variable.

This is with nds2-client 0.16, with python3 on Debian9:

jameson.rollins@zotws17:~ 0$ NDSSERVER=h1nds0:8088 python3 -c 'import nds2; nds2.connection()'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/nds2.py", line 3174, in __init__
    _nds2.connection_swiginit(self, _nds2.new_connection(*args))
NotImplementedError: Wrong number or type of arguments for overloaded function 'new_connection'.
  Possible C/C++ prototypes are:
    connection::connection(connection::host_type const &,connection::port_type,connection::protocol_type)
    connection::connection(connection::host_type const &,connection::port_type)
    connection::connection(connection::host_type const &)
    connection::connection(parameters const &)

jameson.rollins@zotws17:~ 1$