ChangeConfigurationRequest doesn't allow empty values

[sbindzau]

I've tried setting values to "" but that fails with OccurenceViolation as the verification considers empty values to be non-existant.

Removing the "validate" for value is one option, but that seemed too much, so I changed "value" in the struct from string to *string to differentiate the cases. Any objections to that, otherwise I can supply a patch with it (needed to change some tests, too)