Add completions for Manifest commands
Add completions for Manifest commands

This adds the completions for the manifest commands
found in #1902.  This should not be merged until that
is merged, or perhaps this should be cherry-picked
into it.

Signed-off-by: TomSweeneyRedHat <tsweeney@redhat.com>

Closes: #1910
Approved by: rhatdan

projectatomic/buildah,nalind/buildah-1,projectatomic/buildah,projectatomic/buildah,nalind/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,nalind/buildah,rhatdan/buildah,nalind/buildah-1,rhatdan/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,TomSweeneyRedHat/buildah