chroot: only create user namespaces when we know we need them
chroot: only create user namespaces when we know we need them

When running with chroot isolation, only create a new user namespace
when we have mappings to set.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Closes: #1069
Approved by: rhatdan

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