Allow extend rhn.conf file on httpd proxy container

[rjmateus]

We got a report from the community about how to extend the proxy timeout on the httpd container [1], by changing the rhn.conf file.
This file is generated on the fly at container start-up [2] and it doesn't have support to provide the timeout.

We should have a mechanism to enhance this configuration file with all the support properties. The same way we added the ability to pass a tunning file for squid and httpd configuration we should also have the possibility to pass a tunning file for the rhn.conf file.

[1] #9565 (reply in thread)
[2] https://github.com/uyuni-project/uyuni/blob/master/containers/proxy-httpd-image/uyuni-configure.py#L141-L184

[ppanon2022]

As I mention in the discussion, I'm not sure that the rhn.conf proxy.timeout setting is the right one that needs to be changed. However, the wsgi python handler which is timing out does seem to be obtaining a timeout value from a configuration object and then using it when initiating the file download connection to the main Uyuni server. I didn't track all the way back to make sure the rhn.conf file was the source for that object, but it seems reasonable. And well, it's the rhn.conf file - shouldn't it be user modifiable? Otherwise, why not just hardwire those settings into the code?