Use lager instead of fastlog

Update the rebar config erl_opts to include the lager transform
Update error_log calls to use lager directly
  This isn't strictly necessary, as lager will catch error_log calls
  since error_log is just a gen event and we've turned this
  functionality on; however. we get more lager functionality using it
  directly

It's unclear to me if the change to erl_opts here is proper or not. I appreciate some clarification on it. Also see the PR and comments left on erchef here: https://github.com/opscode/erchef/pull/19