[SSSD] [PATCH] DEBUG: Add `debug` alias for debug_level

[Stephen Gallagher]

Our users constantly make the mistake of typing `debug = 9` in the sssd.conf
instead of `debug_level = 9` as would be correct. This happens frequently-enough
that we should just alias it rather than continue to have people make mistakes.

Resolves:
https://fedorahosted.org/sssd/ticket/2999