Fix CMD healthchecks running with /bin/sh

[ben-krieger]

Fixes #23

This change causes ["CMD", "exe", "arg1", "arg2"] style healthchecks to run using the given command directly and NOT using /bin/sh -c.

This is compatible with Podman 1.5.0 and later.

• Unit tests
• Release note

[p12tic]

Thanks for the PR. It looks good. Please add unit test for container_to_args here and a release note to the newsfragments directory (you can look here for inspiration on how release note looks like).