Prefer external.name over x-aws-securitygroup

[ndeloof]

What I did
Add support for networks.external for users to configure an existing security-group
This reduces the (already too large) set of custom extensions we introduced for AWS support

@nebuk89 shall we drop support for x-aws-securitygroup that is not required then ?

/test-ecs

(not mandatory) A picture of a cute animal, if possible in relation with what you did

networks:
  back_tier:
    external: true
    name: "sg-1234acbd"    

[nebuk89]

Yep sounds like a good plan to me

[ndeloof]

update to documentation => #623

[gtardif]

LGTM technically.
Do we want to keep supporting the extension with a warning for some time as the code does here, or remove this extension completely & force users to use external ?