[Uptime] Configure index patterns on the kibana.yml file

[kuisathaverat]

Describe the enhancement:

Currently, the only way to change the index pattern settings for Uptime is by using the UI (or a REST call). The possibility of having these settings in the kibana.yml file.

Describe a specific use case for the enhancement or feature:

We have to provide our users with a simple way to configure a Kibana instance connected to an Elasticsearch cluster configured to make CCS against another Elasticsearch cluster with a large amount of data. The first approach was to configure the connection settings and index patterns for CCS using the kibana.yml. This is possible for APM, Logs, Metrics, and Monitoring, but it is not possible for Uptime.
The alternative is to have some of the configuration settings in the kibana.yml, and a script or something that completes the configuration. This will end in a bad UX due there is no uniform way to configure all the settings in the same place.

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)

[justinkambic]

Post-FF testing LGTM:

Also verified changing the index pattern causes the data shown in the app to change.