Update automatic configuration of SOLR on Pantheon

Attached is a patch that will fix the automatic setup of Solr on Pantheon!

It does the following things:

1. Only enables the Solr server and index if we were successful in posting the schema. This is the main reason this has been broken: it was enabling them even if it couldn't post the schema, which was mostly likely because the Solr addon wasn't enabled in the Pantheon dashboard.
2. Will enable the Solr server and index if the user manual posts the schema later. The pantheon_apachesolr module which we enable on Pantheon will bug them about posting the schema if it wasn't successful at install, and so we hook into the manual process via a hook_form_alter().
3. Makes it so that enabling the Solr server and index doesn't cause the Feature to become overridden. But it will still be shown as overridden on the Search API admin screens, which I think is a good thing! The user can revert back to the "factory settings" but won't pollute the Feature.

My testing so far has looked good!

Changing title since this is only about automatic configuration on Pantheon.

Been testing this for over a month - committed!