Look up HTTPD_USER's UID and GID during installation.

Those values differ among distributions and there is no guarantee that they're
reserved. It's better to look them up based on HTTPD_USER's name.

https://fedorahosted.org/freeipa/ticket/5712

Reviewed-By: Martin Basti <mbasti@redhat.com>

install/share/custodia.conf.template

@@ -5,8 +5,8 @@ auditlog = $IPA_CUSTODIA_AUDIT_LOG
 
 [auth:simple]
 handler = custodia.httpd.authenticators.SimpleCredsAuth
-uid = 48
-gid = 48
+uid = $UID
+gid = $GID
 
 [auth:header]
 handler = custodia.httpd.authenticators.SimpleHeaderAuth