Contexts : do not allow creation of context with type "foo"

[gtardif]

 $ ./bin/docker context create test1 toto
 $ ./bin/docker context ls
NAME                TYPE                DESCRIPTION                               DOCKER ENPOINT                KUBERNETES ENDPOINT                                 ORCHESTRATOR
default *           docker              Current DOCKER_HOST based configuration   unix:///var/run/docker.sock   https://kubernetes.docker.internal:6443 (default)   swarm
test1               toto                                                            

We must check the type and allow only types known by existing backends, or docker ("empty" type)