Add support for wildcard locations for properties and YAML files

[wilkinsona]

No description provided.

[spencergibb]

I assume reading from the filesystem not using k8s api, correct?

[scottfrederick]

I assume reading from the filesystem not using k8s api

Yes, consuming ConfigMaps mounted on the container filesystem using volume mounts.

[mbhave]

We discussed this a bit and it seems like adding a new property source isn't necessary. Here are some things we talked about:

• If the config map results in an application.properties file and you mount it in a config/ directory next to the app, it’ll just work without us doing anything at all.

• If it's mounted at a different location, that location can be changed by setting the SPRING_CONFIG_LOCATION environment variable.

• If you have, let's say both redis configuration and some MySQL configuration, that could end up in config/mysql/application.properties and config/redis/application.properties. @dsyer had once suggested that the ConfigFileApplicationListener could support wildcards.

We've decided to go with the wildcard enhancement and documentation updates as part of this issue.