Support disabling manifest signatures

[mharvey-jt]

It would be useful for us to have unsigned manifests.
This would allow us to remove the signing step from the cvmfs_receiver critical path, and also remove the operational need to maintain and rotate certs.

Proposal:

• Add a configuration option CVMFS_NO_MANIFEST_SIGNATURE_CHECK that will cause manifest validation

  cvmfs/cvmfs/manifest_fetch.cc

  Line 30 in 2333588

  static Failures DoVerify(unsigned char *manifest_data, size_t manifest_size,

  
  to unconditionally skip signature validation, (avoiding the fetch of .cvmfswhitelist, and the X field object).
• Add a configuration option CVMFS_NO_SIGN_MANIFEST that will cause publishing pathways to avoid performing any signing of new manifests, producing a manifest with no X field or signature block, and not trying any X object upload.