added json_attributes option

I added the json_attributes option to load initial attributes just like the knife-ec2 plugin does.
Example:

    bundle exec knife azure server create \
        --azure-dns-name foo \
        --azure-vm-size Small  \
        --azure-source-image b39f27a8b8c64d52b05eac6a62ebad85__Ubuntu-13_04-amd64-server-20130928-en-us-30GB \
        -m 'West US' \
        -ssh-user ubuntu \
        -E production \
        -r "role[mongodb_arbiter]" \
        -j '{"mongodb":{"role":"arbiter","cluster":"mongo-cluster-A","replicaset":"rs1"}}'