Clarify upgrade process for those going to Bitbucket Data Center and Elasticsearch

If a user is upgrading an existing instance for Data Center, we tell them to follow the standard Bitbucket Server upgrade guide.

When they do so, Elasticsearch is automatically installed and is running. If they follow the same process for a new node, they now have two instances of Elasticsearch running.

Requested Changes

1. We need to clarify for user upgrading for Data Center to choose the "Upgrade existing Data Center installation" in the Bitbucket installer. And if they are installing a new node, to call out they should choose "Install new Data Center installation". Page Installing Bitbucket Data Center
2. For step 5 of the Installing Bitbucket Data Center page, we need to clarify why a user needs to set up a separate Elasticsearch instance. Users will need to know that when using the "Install/Upgrade Data Center" options that Elasticsearch will not be bundled/installed.
3. On the Install and configure Elasticsearch page, we need to clarify that Elasticsearch is not automatically installed for Data Center users.