Add check of tarball content

[mgoerens]

Checks that the tarball contains a unique directory named after the chart. This directory must contain a Chart.yaml file. The directory may contain additional files or folders. No other files or folder should be placed at the root of the archive.

Fix #342

Running the check on all tarballs currently in the charts repo, I'm getting some (expected) errors with the redhat charts that were renamed:

❯ python check_tarfile_content.py -d tarball_dir/
Error with tarball redhat-data-grid-8.3.0.tgz: [ERROR] Incorrect tarball content: expected a redhat-data-grid directory
Error with tarball redhat-data-grid-8.3.1.tgz: [ERROR] Incorrect tarball content: expected a redhat-data-grid directory
Error with tarball redhat-data-grid-8.4.0.tgz: [ERROR] Incorrect tarball content: expected a redhat-data-grid directory
Error with tarball redhat-data-grid-8.4.2.tgz: [ERROR] Incorrect tarball content: expected a redhat-data-grid directory
Error with tarball redhat-data-grid-8.4.3.tgz: [ERROR] Incorrect tarball content: expected a redhat-data-grid directory
Error with tarball redhat-semeru-transition-1.0.0.tgz: [ERROR] Incorrect tarball content: expected a redhat-semeru-transition directory

For instance the redhat-data-grid-8.3.0.tgz tarball contains a data-grid directory instead. Newer versions of this chart contain valid tarball.

For redhat-semeru-transition-1.0.0.tgz, there is no newer version than 1.0.0

[jsm84]

Review complete.

LGTM