Makes microversions config option clear about 'None'

If StrOpt option is explicitly set as None, oslo config
will return it as string "None" which should be invalid
configuration.
Example-
   min_microversion = None will be end up with 'None'.

This commit makes that clear in config and raise exception
for the same.

Partially implements blueprint api-microversions-testing-support

Change-Id: I81e86faca6f8c0ffb7da22154a62236ac25cf0c0
3 files changed