Add ability to exclude environment variables when initializing AbstractEnvironment

[btiernay]

In some environments, common environment variables may be set that may effect application binding in undesirable ways. A common example is the MAIL environment variable in many linux systems. If this is set, it will likely cause a binding exceptions in a @ConfigurationProperties class that uses the mail namespace. To avoid such scenarios, it would be desirable to add a "black list" of environmental variables to ignore.

[btiernay]

Seems to be related to #3427, #3832

[philwebb]

I think this is a duplicate of #3427. It's been on the list for a little while but it might be something we can't fix properly until Spring Boot 2.0.