(QENG-1414) Allow installation of OSX using simplified agent

OSX has been a supported simplified platform since its release, but
beaker did not support this deployment pattern. This change makes it
work.

Conflicts:
	lib/beaker/dsl/install_utils.rb
Merge pull request #507 from branan/osx_simplified_support

(QENG-1414) Allow installation of OSX using simplified agent