#345 Nginx: creation and initialization of *hosts.conf file

Note: I modified the `NgnixTest`, changing from `TemporaryFolder` into a `@BeforeClass` mechanism in order to ensure that the name of the temporary folder stays the same across the execution of the test class. The problem was that `Nginx` derives its config directory from the `Thindeck-Loadbalancer-Directory` manifest entry. Since the `Manifest.append()` and `Manifest.read()` methods are static, this causes a race condition when the tests are executing concurrently, since `TemporaryFolder` creates a different folder for each method that is executed. As a result the tests fail intermittently since they're stepping on each other's data. This is likely to have caused the error in #362 as well.