Add configuration of solr port, host and base with zcml

This is a port from [ftw.solr](https://pypi.python.org/pypi/ftw.solr/) to configure solr connection settings through zcml.

The zcml configuration takes precedence over the configuration in the configlet. If there is no zcml configuration, or the values configured in zcml don't create a connection, the connection is created from the configlet values.