(QENG-713) add 'puppet_tmpdir' method

Add a 'puppet_tmpdir' method in Unix::File that creates temporary
directories which are owned by the puppet user.
add test case and docs for 'puppet_tmpdir' method
use DSL to access --configprint value
Adds create_tmpdir_on DSL method.

Signed-off-by: Wayne <wayne@puppetlabs.com>
Update to create_tmpdir_for_user_on, add spectests

This commit as spec tests which include several additions to spec/mocks.rb to
support the use of `host.tmpdir` and `execute` when using FakeHost. In support
of these is the FakeCommand which works around the fact that these FakeHost
commands don't have access to the same objects as the context in which
FakeHost.exec is used.

Also removes test for Unix::Host.puppet_tmpdir

Signed-off-by: Wayne <wayne@puppetlabs.com>