Rename snowplow_collector_uri to snowplow_collector_hostname

Problem

The application_settings.snowplow_collector_uri column is incorrectly named and could lead to be incorrectly set. The name suggests a URI/URL is expected, but in fact it's a hostname.

Proposal

• Rename application_settings.snowplow_collector_uri secret to application_settings.snowplow_collector_hostname

• should also be renamed / validation updated.