fix building docs from tarball

Closes #1945 (closed) a second time.

• 801ec5ac fix building docs from tarball

  • Move tools/* EXTRA_DISTs inside tools/Makefile.am because that's where they belong.
  • Add check-messages.py to EXTRA_DIST in tools/Makefile.am to allow someone who downloads the tarball to regenerate the entire message documentation just like before.
  • Add debug-messages.rst and kea-messages.rst to the tarball so that they are not forced to use check-messages.py to regenerate message documentation.
  • Add singlehtml to all rule. It is the only type of docs that are not built by default.

Fixes doc and pkg jobs:

• https://jenkins.aws.isc.org/job/kea-manual/job/doc/20/
• https://jenkins.aws.isc.org/job/kea-manual/job/pkg/420/