Coerce the datatype property into an atom.

When creating a bucket-type from the command-line, the `datatype`
property might not be an atom, so we should try to convert it to one
before passing it to the CRDT code.
Move the datatype property coercion into riak_kv_wm_utils:erlify_bucket_prop/1.
Merge pull request #698 from basho/bugfix/sdc-datatype-bucket-prop-format

Coerce the datatype property into an atom.