[EPM] await call to InstallPrebuiltComponentTemplates

[skh]

During the installation of prebuilt index templates (v2), we install prebuilt component templates first, and then the templates which use these component templates. This happens here: https://github.com/elastic/kibana/blob/master/x-pack/plugins/ingest_manager/server/services/epm/elasticsearch/template/install.ts#L23-L24

We're probably missing an await here to ensure that the component templates are present when they are used. This wasn't a problem during testing but looks brittle.

Also, prebuilt templates are used by the base package, which is used for the first setup of Ingest Manager, so this has the potential of making the whole of Ingest Manager unusable.

(Thanks @neptunian for spotting it.)

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[skh]

This actually happened to me now during testing, and it results in server crash due to an unhandled promise rejection.