Set kibana reporting encryption key

[anyasabo]

Similar to the session encryption key, we should set the reporting key xpack.reporting.encryptionKey by default so that in progress reports can be resumed:

Set to any text string. By default, Kibana will generate a random key when it starts, which will cause pending reports to fail after restart. Configure this setting to preserve the same key across multiple restarts and multiple instances of Kibana.

I noticed this warning log message when verifying the other encryption key functionality:

{"type":"log","@timestamp":"2019-12-17T16:12:58Z","tags":["reporting","warning"],"pid":6,"message":"Generating a random key for xpack.reporting.encryptionKey. To prevent pending reports from failing on restart, please set xpack.reporting.encryptionKey in kibana.yml"}

Docs:
https://www.elastic.co/guide/en/kibana/current/reporting-settings-kb.html#general-reporting-settings

[anyasabo]

Ditto xpack.encrypted_saved_objects.encryptionKey