SOLR connection parameters should be configurable properties

[datadavev]

See /wiki/Notes for some comments on configurable properties of the gateway service. Text repeated here:

There are currently a couple of parameters (and possibly more over time) that should be stored as configuration options, ideally editable through a web interface such as the Django admin interface.

Currently, config parameters include:

• The {base} of the gateway service URLs
• The SOLR service host, port, and base URL
• Names of required fields such as:
  • The document unique ID field
  • The document last modified field

Options such as these should be set at least in settings.py, and ideally through the administrative interface.

[coreyo]

To support multiple data backends and backend types, settings should be defined as a dictionary of key/values for each of these parameters. This will ensure flexibility of different parameters as we implement different backend types.