(MODULES-1189) force module install in spec_prep

When `puppet module install` runs it checks if the module is already
installed in the modulepath. If it is it refuses to install the module
even to a different target-dir.

This forces the module install to target-dir which bypasses the unnecessary
check.
Merge pull request #67 from 3flex/force-module-install-to-fixtures-directory

(MODULES-1189) force module install in spec_prep