Fix RPC configuration file test case

The relevant test case test_rpc_with_configuration_file_overwriting_rpc()
was actually designed around the broken behaviour. It was only working
if a previous configuration file (set via environment variable in this
case) and the RPC configuration file have the same name. The test case
which tests that RPC configuration file settings are overwriting direct
RPC settings now makes sure that no other configuration file is set via
the environment variable. If it would be set, the test case would still
succeed, even with the previous patch applied. Which is and which was the
correct behaviour.

So the main reason for this change is to test the more likely use case
that the environment based configuration file and the RPC configuration
file have different names.

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>