Upgrade Assistant should set index.mapping.single_type=true

[immon]

Describe the feature:

Upgrade Assistant in 5.6 should set index.mapping.single_type to true after reindexing .kibana. It will help to avoid issues when an old Kibana (eg 5.6.1) adds multiple types to .kibana-6 and breaks it.
New Kibana (eg 5.6.10) refuses to start with the following error:

{"type":"log","@timestamp":"2018-07-17T08:09:16Z","tags":["status","plugin:[email protected]","error"],"pid":1,"state":"red","message":"Status changed from red to red - [illegal_argument_exception] mapper [version] cannot be changed from type [long] to [integer]","prevState":"red","prevMsg":"[remote_transport_exception] [soJvHr6][10.6.220.234:9300][indices:admin/mapping/put]"}

We already set single_type to true when creating .kibana index #11451 but not after Upgrade Assistant has reindexed .kibana to .kibana-6

Describe a specific use case for the feature:

/cc: @jbudz

[tylersmalley]

Closing this as no further changes are being made to the upgrade assistant in 5.6 and Kibana now has it's own index migrations.