I had some issues in geterogenius environment when reverse proxing to
roundup-tracker while authentication is external.

To solve this i propose a little patch (included).

This patch do the next things:

1. From user point of view:
In config.ini new option "uid_variable" is appear in section [web] with
following description:
"
In case of http_auth option is set to 'yes' this option
controls which envirionment variable is used to determine
user login name. In most cases your web server supply
the REMOTE_USER variable, which is default value.
So, change this option only in special cases. 
Note: HTTP-request options is available as corresponding
'HTTP_' prefixed variable.
Default: REMOTE_USER
"

This option is compatible with any previous configuration and doesn't
make any change in roundup-tracker behavior.

2. To internals of roundup-tracker:
a) The option is added in configuration.py
b) Change behaviour of determining user during request. Now it will
search "uid_variable" variable instead of hardcoded REMOTE_USER.
c) Make all HTTP-request options available in internal environment as
"HTTP_" prefixed variable, not just few hardcoded.
This made with 5 lines stolled from WebWare
http://www.webwareforpython.org/. 1 line comment in roundup-code note
this regrettable fact.

Patch included.