current search_api_solr module has no solr-conf directory

The current setup procedure relies on the solr-conf/7.x directory to be initially mapped into the solr container. This does not work with the current search_api_solr module.

We should either

use the official drush search-api-solr:get-server-config to load the current config and deploy it in solr (more complicated).
check in an initial solr config like we did with older search_api_solr.

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