CHEF-4725: Validate 'log_location' setting on log initialization

Previously, a Mixlib::Config setter method verified that 'log_location'
was set to a writeable file or a valid IO stream. Due to the way
Mixlib::Config handles `default` and `merge!`, this setter method would
not fire if the 'log_location' was left unconfigured or if it was set
on the command line, resulting in inconsistent error handling.

This commit moves the validation logic out of the configuration layer
and into the log initializion layer. This ensures that error handling is
consistent, regardless of where the 'log_location' setting is
configured. Validation logic is also simplified, relying on the
MonoLogger class to open and configure any necessary IO streams.
CHEF-4725: Disable buffering on all log streams

Centralize handling of log buffering to ensure it's always disabled.
Previously, an unconfigured 'log_location' setting would erroneously
fall back to a default STDOUT stream with output buffering enabled.
Fix indentation of rescue clause
Merge branch 'CHEF-4725'
Add indirection to allow switching policy builder implementations
Extract "expand node" policy builder to new file
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
Update error messaging in Policyfile policy builder
Fix incompatibilites between node expansion and policyfile
Fix Policyfile tests for ruby 1.8
Merge branch 'policyfile-support'

https://tickets.opscode.com/browse/CHEF-4984
Return node from Chef::Client#build_node
adding validation of content-length field
WIP: hacking on ugly spec tests
add counting content-length for streaming requests
WIP: partial rewrite of base specs on master
more test refactoring
rspec modernization
add specs for CHEF-4502 behavior