(QENG-989) install_puppet_from_gem fails on Solaris

Trying to install puppet from gems would fail on both Solaris 10 and
11. Solaris 10 now has pkgutil and gem symlinked (ln -s) to /usr/bin.
Both Solaris versions have puppet-related gems symlinked to /usr/bin
after installation. This is to avoid clobbering the PATH.
This PR introduces using the "EXECUTABLE DIRECTORY" portion of `gem environment`. The "/does/not/exist" is just a dummy value because I did not want to use an empty string (`""`) and this the usual return type is a directory.