Fix unsafe interpolations

[smortex]

We now have puppet-lint-check_unsafe_interpolations and it detect some
issues. Fix them.

[smortex]

Unfortunately due to puppetlabs/puppet-lint-check_unsafe_interpolations#39 we are still required to disable the check.

[smortex]

I updated the PR to use stdlib::shell_escape() in favor of shell_escape() (deprecated).

I also removed the comments to disable the check to avoid the false positives because:

1. This check is not enabled by default;
2. I opened Fix safe interpolation detection puppetlabs/puppet-lint-check_unsafe_interpolations#48 to fix the bug.

When a version of puppet-lint-check_unsafe_interpolations that can spot shell_escaped interpolations is released, we will be able to consider its inclusion at the voxpupuli level and this module should not raise any warning when this happen.