Add prefix support for environment variables

[lucassaldanha]

Creates an environmentPrefix property that is used to filter the
SystemEnvironmentPropertySource's map of environment
variables and create a new one only with the prefixed properties.
This new map is used on a new SystemEnvironmentPropertySource
that replaces the former one.

Fixes #3450

[lucassaldanha]

@philwebb I didn't understand why the build failed. Looks like some error in Travis CI worker... Do you know what happened?

[snicoll]

I don't think that's what we had in mind. We shouldn't change the existing PropertySource as anybody may want to look it up manually (either via the Environment or @Value).

We thought of applying that constraint for binding so that not the full OS environment is available as it may actually break it (see the discussion on the original issue).

My best guess is that that PropertySource replacement should occur when @ConfigurationProperties POJO are bound to the environment (there's a PostProcessor that does that). Do you want to give that a try and submit a new PR? (please push force on your branch rather than creating a new PR)

[lucassaldanha]

@snicoll Ok! I'll adjust the code to match the behavior you described.

[lucassaldanha]

@snicoll I'm not sure if this is the best way to configure the environment prefix. I'd like some opinions on that.

[lucassaldanha]

@snicoll I believe that now it does what is is expected to do (considering the prefix only on binding).

I made a line note about the way that the environment prefix is being configured. Because I'm not sure if it's a good idea using the system properties to do that. Need some opinions.

[snicoll]

@lucassaldanha this PR was automatically closed because you made it from master and your changes went away when you push force on your fork. If you still want to contribute this change, please create a PR from a branch and not master. Thanks.