Correct template path filter logic to only include regular files

[nabertrand]

It looks like the original logic in PDK::Module::TemplateDir.files_in_template was created to filter paths that weren't regular files, but that logic was altered in commit 7facba3#diff-f1961fa3f901b19ee585c1962e910d5eL193 and the filter no longer works as exptected. If a directory other than "spec" appears in the moduleroot or moduleroot_init folders, pdk new module fails with the error

pdk (FATAL): Failed to render template '<directory>'
ArgumentError: '/tmp/pdk-templates20180614-14695-18x9jy1/moduleroot_init/<directory>' is not a readable file

This patch restores the original behavior.

[coveralls]

Coverage decreased (-0.01%) to 92.785% when pulling 0238158 on nabertrand:template_dir_fix into c9fb0ff on puppetlabs:master.

[nabertrand]

It looks like this is same issue as #445 but solves the problem slightly differently than #512. #512 will create a possibly unbalanced dirlocs list @ https://github.com/puppetlabs/pdk/pull/512/files#diff-f1961fa3f901b19ee585c1962e910d5eL216

[scotje]

Thanks for your contribution!

[thomas-illiet]

Thanks !!!