Make property binding failure handling configurable

[wilkinsona]

Currently, handling of property binding failures is hardcoded to ignore failures for properties sourced from the system environment and system properties. We should look to make this configurable via a setting on SpringApplication so that users can tighten or relax the rules as required.

Due to RelaxedDataBinder being used in multiple places, this will require some refactoring to ensure that the configuration can be applied in a single, centralised location and used everywhere.

[mbhave]

@wilkinsona I'm not sure how the old one worked but this might no longer be the case with the new binder. what do you think?

[wilkinsona]

@mbhave I'm not sure. This issue was opened to make the changes added in a0870c1 configurable. No one's actually asked for it so it's certainly tempting to say that we don't need it

[mbhave]

I think with the new stuff, if ignoreUnknownProperties is set to false, binding failures will cause the app to fail hard, regardless of where the properties came from. We should consider if we want the default behavior to be the same as what was introduced in a0870c1 (ignore binding failures for
properties that come from a source outside of the application’s control,
namely environment variables and system properties).