Move EnginioBackendManager url into EnginioClient

This way it would not be possible to accidentally execute some
of operations on a different host.

Change-Id: I51ed0406ffab9be5575e85d68792ccb2da49e9bf
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Remove EnginioBackendManager::setServiceUrl

The url is set trough environment variable

Change-Id: Idfb21623908f0f0f66be460dffa34b6c6f1a9935
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>