Add Policyfile-based policy builder for chef-client

NB: This is experimental and a bit of a hack. There is no server-side
support, so interactions with the server are coded in a "compatibility
mode" using data bag items and Chef 11 style cookbook version APIs.

The Policyfile PolicyBuilder uses a single document from the server as
the authoritative cookbook version set and expanded run list. Enabling
it will disable support for environments and roles. In addition,
chef-solo and override run lists are currently unsupported, though they
could be supported in the future.
Allow policyfile feature to be enabled in config