Integrate better with the spring-boot-zalando-stups-tokens starter

[BGehrels]

At the moment, the user needs to define the token url and the scopes in a different format than for the rest of her application if she uses spring-boot-zalando-stups-tokens.

We should use the AccessTokens bean initialized by spring-boot-zalando-stups-tokens if it already is present.

[ePaul]

This is needed when deploying your application in Zalando's Kubernetes infrastructure, as there you just get your token into some file (and the app doesn't fetch it from some URL, or gets to decide which scopes to have).

An application can currently configure it like this, given that accessTokens is a bean implementing the AccessTokens interface, e.g. from tokens-spring-boot-starter.

@Bean	
public org.zalando.nakadiproducer.AccessTokenProvider nakadiProducerAccessTokenProvider() {
        return () -> accessTokens.get("nakadi");	
}

Our library should be able to do this by itself, possibly with a configuration property saying which token to use from the AccessTokens object, possibly with a fallback to just nakadi.

[ePaul]

Idea from @BGehrels: Only reuse existing AccessTokens bean if it exists and there is no nakadi-producer.access-token-uri defined (and we are not already using the existing Fahrschein bean from #85).

[whiskeysierra]

The tokens library, and all supporting libraries, are not really needed for k8s anymore.