(RE-399) make build object properties accessible

It would be useful to have access from the rake entry point to all of the build
properties individually. This commit makes it so, by creating a new rake task,
pl:print_build_param[:param], where the passed parameter is the one to print.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #257 from MosesMendoza/RE-399/master/make_param_print

(RE-399) make build object properties accessible