Enable explicit clearing of values

[dmlloyd]

Description

As a:

• Application user/user of the configuration itself
• API user (application developer)
• SPI user (container or runtime developer)
• Specification implementer

...I need to be able to:

have a way to specify that a property is explicitly cleared.

...which enables me to:

• clear properties on the command line or system environment for testing and debugging purposes e.g. -Dsome.property=
• reuse configurations by overlay, even if the underlying configuration specifies a configuration value I do not want to have set