Add warning about Debian/Ubuntu augeas packaging bug

This adds a warning to the docstrings for both the augeas state and
execution module, explaining the issue discovered in #6082. For more
information, see the comments here:

https://github.com/saltstack/salt/issues/6082#issuecomment-27234356
Merge pull request #8153 from terminalmage/issue6082

Add warning about Debian/Ubuntu augeas packaging bug