Honor `:node-type`, `:bridged-interface` and `:local-interface` in `node-spec`

Fixes #10.

These entries go under `:hardware` in the node-spec. These entries
will override any provider-specific network settings and the defaults
values when none is specified.

With this fix, pallet now will honor these directives. You can pass
`:node-type` (`:bridged` or `:local`) to the node spec, and also which
network interface to use in each case via `:bridged-interface` and
`:local-interface`, e.g:

```clojure
(pallet.api/node-spec :image {:image-id :ubuntu-12.04}
                      :hardware {:network-type :bridged
                                 :bridged-interface "en0: Ethernet"}))
```
Merge branch 'develop' of github.com:pallet/pallet-vmfest into develop