buildah: set the HTTP User-Agent
buildah: set the HTTP User-Agent

Closes: https://github.com/projectatomic/buildah/issues/573

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Closes: #814
Approved by: rhatdan

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