Support namespaced logins for quay.io
Support namespaced logins for quay.io

Service accounts (a.k.a. robots) in `quay.io` are forcably namespaced
to the user or orginization under which they are created.  Therefore,
it is impossible to use a common login/password to push images for
both `skopeo` and `containers` namespaces.  Worse, because the
authentication is recorded against `quay.io`, multiple login sessions
are required.

Fix this by adding a function definition which verifies non-empty
username/password arguments, before logging in.  Call this function
as needed from relevant targets, prior to pushing images.

Signed-off-by: Chris Evich <352d8596d782a469eafa66a46434e15a449859ce@redhat.com>

mtrmac/skopeo,mtrmac/skopeo,mtrmac/skopeo