Allow running as arbitrary UID for unprivileged Container Environments (OpenShift)

[rezemble]

Is your feature request related to a problem? Please describe.

In order to be able to run Icinga in a context with indeterminate User IDs, such as OpenShift, the group-injection logic from icinga.cpp seems to be an obstacle:

  if (!pw) {
  	if (errno == 0) {
  		Log(LogCritical, "cli")
  			<< "Invalid user specified: " << user;
  		return EXIT_FAILURE;
  	} else {
  		Log(LogCritical, "cli")
  			<< "getpwnam() failed with error code " << errno << ", \"" << Utility::FormatErrorNumber(errno) << "\"";
  		return EXIT_FAILURE;
  	}
  }
  // also activate the additional groups the configured user is member of
  ...

Describe the solution you'd like

From my testing, activating additional groups in such a context is superfluous, since there are none; since the process is run as an arbitrary UID with GID 0, removing the if (!pw) { check and instead wrapping the group activating logic in

if (pw) {
 // also activate the additional groups the configured user is member of
 ...
}

works fine

Describe alternatives you've considered

I've considered wrapping the image with dynamic user and group renaming, but this adds unnecessary complexity and image layers