fix systemd for Ubuntu 14.10

It has systemctl installed on the system but does not have systemd
wired up as init by default and trying to use systemctl throws
exceptions that breaks everything.

Also add a big banner warning because I'm sure someone is going to
think that the shortest past to fixing priority bugs is hacking up
what gets reported from these modules, rather than fixing the bugs
in the downstream logic.
correct list-unit-files + refactoring
extract which to a mixin, use for systemctl
systemctl path fixes, rspec modernization, etc

- systemd provider uses `which` mixin to determine systemctl path
- let-ification of spec tests for system provider
- fix some stubbing and mocking of current_resource that was confusing
  and useless
add some more debug output and comments
polishing provider_resolver

* makes provides? more correct for linux services
* makes supports? more targetted for linux services
* remove provider_resolver from the run_context
* fix timestamped deploy spec
* add more specs
syntax fixes for grepping output
spec fixes
rspec 3 fixes
Update md files.
Merge pull request #2336 from opscode/lcg/12-systemd-fixes

fix systemd for Ubuntu 14.10