(PUP-3069) Add default manifest settings

Some Puppet installations do not use environment specific manifests
(particularly when an ENC is in use), however may still have global
setup required for all environments. For these installations it is
convenient to have a :default_manifest setting specified rather than
copying them manifest into every environment.

There are also installations which have a separation of roles between
users mainting the puppet master, puppet.conf and enc, and users making
per environment module changes. In these cases some Puppet installations
prefer to restrict the use of per environment manifests.

These two use cases were handled by setting the global manifest to a
non-interpolated path while setting the global modulepath to a path
interpolated with $environment.

With these settings being deprecated, directory environment
installations did not provide a means of preserving this functionality.

This patch addresses this by providing two new settings:

* default_manifest - is now the fallback manifest path if non is
specified in a given environment's environment.conf. It defaults to the
environment root relative path './manifests' which is the same default
that was previously hardset in early version of directory environments.
* restrict_environment_manifest - if set to true, enforces that
environments cannot define their own manifest setting, and validates
that default_manifest is set to an absolute path.

It is an initialization error if default_manifest is set to a string
that has an '$environment' to be interpolated within it.

We also log an error if restrict_environment_manifest is true and the
loaded environment's environment.conf has a manifest setting that
differs from the default_manifest setting.
(PUP-3069) Add spec test for new default options

Added tests for checking that default_manifest behaves properly when
given $environment. Also when restrict_environment_manifest is true with
a non-relative default_manifest.

This commit also fixed the hook on the :default_manifest setting to
validate that $environment was not present when setting the value.

(WIP) Added spec tests for new default options

(WIP) Fixed tests for Puppet::Settings::BaseSetting#value
(PUP-3069) Halt compile if environment has manifest conflict

Henrik brought up that with the new restrict_environment_manifest
setting set true, authors of modules in a particular environment
may be surprised by a manifest they have specified in their
environment.conf not being pulled in, leading to bad behavior when a
catalog is executed.  Although this is logged on the master, compilation
delivery and catalog execution is not prevented.

This commit will raise an exception on the master prior to catalog
compilation if it detect's a conflict between manifest settings in the
requested environment.