Handle /root not existing in image

Aufs workaround assumed /root existed in the image. It might not.

[#83655858]
Do bind mounts in parent

Binding in child appears to conflict with user namespacing changes.

[#83654538]
Fix typo in test

[#83467000]
Add bind_mounts integration tests

Tests all combinations of Host/Container; Privileged/Unprivileged; Read-only/Read-write.

Change ifrit.Envoke to ifrit.Invoke.

Document binPath directory placement in test suites.
Without this relocating a suite causes an obscure failure.

[#83769328]

Signed-off-by: Glyn Normington <gnormington@pivotal.io>
Merge branch 'master' into bind-mount-integration-tests

[#83769328]

Signed-off-by: Glyn Normington <gnormington@pivotal.io>