[WiP] - JENKINS-24702

convert legacy credentials stored as plain text (including Oauth token :-\) to use credentials-plugin.
Maybe AccessTokenCredential is generic enough for this to be defined by some oauth-credentials plugin, not sure how portable this is and if this would make sense.
@jglick I discussed this with @stephenc and he confirmed the server URL is part of the credentials instance in such a context. I had preferred I can pass the current server domain to the credentials creation form so this is not required, but still can generate / validate token.