Make context orchestrator field optional

[aiordache]

Signed-off-by: aiordache anca.iordache@docker.com

When loading a context without the StackOrchestrator field set, we get an error.

Traceback (most recent call last):
  File "main.py", line 64, in <module>
    test_get_docker_context()
  File "main.py", line 34, in test_get_docker_context
    ctx = ContextAPI.get_context("dev")
  File "/src/github.com/docker/docker-py/docker/context/api.py", line 97, in get_context
    return Context.load_context(name)
  File "/src/github.com/docker/docker-py/docker/context/context.py", line 62, in load_context
    name, orchestrator, endpoints = Context._load_meta(name)
  File "/src/github.com/docker/docker-py/docker/context/context.py", line 88, in _load_meta
    metadata["Name"], metadata["Metadata"]["StackOrchestrator"],
KeyError: 'StackOrchestrator'

Made this field optional so it won't raise error on load.

[ulyssessouza]

LGTM