Support multiple SOLR schema versions

[amercader]

At some point the SOLR schema used by CKAN will need to be changed and the changes won't be always backward compatible, so we need a mechanism to support instances that use older versions of the schema. This process will involve:

• Better documentation on how to deploy SOLR, as single instance or multicore
• A convention on how to tag schema versions and SOLR endpoints to ensure that CKAN instances point to the correct SOLR URL.
• (If possible) Checks in CKAN core that deal with different schema versions.