Fixed certificate/key paths

Right now, generated certificates/keys doesn't have any fixed filename. They are created within certdir, their filename is unspecified. This causes some difficulties with packaging and integration.

After some digging in source code, I see they take <SERIAL>.(key|pem|csr) filename. But I think it's wrong to rely on that and would require some hacking to integrate that paths with sentinel-proxy. Packages on router in general rely on fixed and well-known paths.

As discussed with @mprudek, I would suggest to create the have fixed and well known name. Either by some additional argument or by default.