Add bash completion for 'dnf module' (RhBug:1565614)

[inknos]

BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1565614

[packit-as-a-service]

There was an error while running a copr build:

{'chroots': ['Such chroot is not available: fedora-29-x86_64']}

You can re-trigger build by adding a comment (/packit copr-build) into this pull request.

[kontura]

In PRs #1100 and #1102 there are comments that command names and options could be generated from argument parsers instead of using hard-coded strings.

However it could take a while before its finished so in the mean time I think this is useful.
Thanks!