Merge branch 'ocsp-responder-index'

Adds support for multiple OCSP responders in `pki --ocsp` and one that
is based on OpenSSL-style index.txt files.  The parser for these files
also accepts simplified files that only specify the status, serial number
and optional revocation date/reason.  The OCSP test scenarios are also
updated to use this OCSP responder including one that shows the multi-CA
capabilities of the --ocsp command and the --index option.