Fix #643 - Update of opendkim lens to match current opendkim.conf file format

[doc75]

Modification done to match possible format of /etc/opendkim.conf file:

• Allow string values to start with numbers [0-9] like in AutoRestartRate 10/1h
• Allow string values to contain '/' like in TrustAnchorFile /usr/share/dns/root.key
• Allow boolean values to start with uppercase letter: False, True, Yes, No
• Add support of parameter LogWhy with boolean value.

[doc75]

I added a test to check the modifications, but I cannot see the opedkim tests being replayed while launching make check.
If you have any idea, let me know how to replay it, I'll be more than happy to replay the tests).

[raphink]

The tests are played in CI here https://travis-ci.org/hercules-team/augeas/builds/574425620#L3880