Add the "client_credentials" grant type to the default admin client

[pletessier]

See issue gravitee-io/issues#1281

[tcompiegne]

Hi @pletessier, don't you think we should also update existing admin client ?

Your code only apply if the admin client doesn't exist.

[pletessier]

Hi @tcompiegne ,
From my point of view, setting the grant_type at init is enough. I just need that to be able to create an access_token the first time. If user changes are made after, I suppose that Gravitee shouldn't overwrite them. It would be confusing.

[tcompiegne]

All right, according to our contributing guide, can you sign the CLA, and I will accept your PR.

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Please sign our Contributor License Agreement (CLA) and send it to contact@graviteesource.com. For any code changes to be accepted, the CLA must be signed.

Thanks.
Regards.

[tcompiegne]

👍