Improving tempest tests configuration.

Applied changes
-  Adding correct private network name to tempest.conf
-  Adding controller hostname to tempest.conf
-  Decreasing "max_instances" parameter in a [stress] section of tempest.conf

Affected tests: 
- Scenario tests class (Series of steps where complicated state requiring multiple services interaction)
- Stress tests class

Notice: This pull request contains changes from pull request #62 (which is still not merged) in order to work properly.