allow verify/2 to accept atom for encryption type

ping @opscode/server-team 

In order to work around an issue in erlang compiler which prevents us
from specificying {d, value, <<"abinary">>}.  as a compiler option in rebar.config,
we needed to set default encryption types to be an atom. - this change
allows verify/2 to convert a received binary string to atom to prevent
match failures.
We are leaking them, but based on the compilation mode - that gets passed in from ```-define```d  value.   
I'd like to actually contain all of this within chef_user and/or chef_objects - there's nothing webmachine related in chef_wm_password -  but that's for another iteration.