Make clear in docs that DBs should be installed externally for production use

Current documentation with the instructions for installing 3scale, shows the installation using internal databases as the "Default deployment configuration" (section 2.7.1). This can cause confusion because deployments using internal DBs are only suitable for proof of concept (PoC) or evaluation.

One proposed solution is to merge sections 2.7.1 and 2.7.2 into a single POC or Evaluation deployment instructions with two flavours: normal and lightweight. Then make it clear that the configuration for external databases is the standard deployment option and the way to go in production environments.