registry: make certain headers optional

For a pull-only, static registry, there only a couple of headers that
need to be optional (that are presently required.
* X-Docker-Registry-Version
* X-Docker-Size
* X-Docker-Endpoints

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
registry: Info collection

roll version and standalone information into the _ping. And to support
Headers they are checked after the JSON is loaded (if there is anything
to load). To stay backwards compatible, if the _ping contents are not
able to unmarshal to RegistryInfo, do not stop, but continue with the
same behavior.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
static_registry: update the test for the new struct

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Use proper scheme with static registry
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4607 from vbatts/vbatts-static_registry

static registry support
Cleanup existing controllers when cleanup fails mid-way.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Merge pull request #5474 from crosbymichael/use-proper-scheme

Use proper scheme with static registry
docs: DisableNetwork -> NetworkDisabled

Docker-DCO-1.1-Signed-off-by: Samuel Reis <srijs@airpost.net> (github: srijs)
Merge pull request #5476 from rjnagal/libcontainer-fixes

Cleanup cgroups on Set failures
Merge pull request #5475 from srijs/patch-1

docs: DisableNetwork -> NetworkDisabled
Update working-with-docker.md
Merge pull request #5478 from amangoel/patch-1

Update working-with-docker.md