Add prefix support for environment variables

[philwebb]

Currently environment variables get bound to @Configuration properties directly (for example SERVER_PORT to Server.port). This can cause problems when trying to set ignoreInvalidFields or if you want to run two different boot apps as the same user.

@gregturn suggested that we consider supporting a prefix when binding. For example:

springApplication.setEnvironmentPrefix("MYAPP_");
...

Which would then bind MYAPP_SERVER_PORT rather than SERVER_PORT.

[snicoll]

I like that.

[lucassaldanha]

+1 for this feature!

[lucassaldanha]

Here is what I thought:

At the SpringApplication.configurePropertySources(..) we can filter the environment.getSystemEnvironment() map and replace it with a new one just with the properties which keys start with the prefix.

I though about something like this:

protected void configurePropertySources(ConfigurableEnvironment environment, String[] args) {
    ...
    if(this.environmentPrefix != null && !this.environmentPrefix.equals("")) {
        filterSystemEnvironmentPropertySourceByPrefix(environment);
    }
    ...
}

private void filterSystemEnvironmentPropertySourceByPrefix(ConfigurableEnvironment environment) {
    final Map<String, Object> systemEnvironment = environment.getSystemEnvironment();
    final Map<String, Object> prefixedSystemEnvironment = new HashMap<String, Object>(systemEnvironment.size());
    for(String key : systemEnvironment.keySet()) {
        if(key.startsWith(environmentPrefix)) {
            prefixedSystemEnvironment.put(key.substring(environmentPrefix.length()), systemEnvironment.get(key));
        }
    }   

    environment.getPropertySources().replace(
        StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME,
        new SystemEnvironmentPropertySource(StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, prefixedSystemEnvironment));
}

What do you think? There are any collateral damage doing this? I have tested myself and it seems o work quite well. If you want I can make a Gist or a PR with my code.