state: make InferEndpoints take varargs

Most of the callers, usually the tests, were having to construct a []string, just to satisfy the contract of the method.

Make the method take a varargs ...string which is consistent with other Relation methods.
Merge branch 'master' of https://github.com/juju/juju into 198-state-infer-endpoints-takes-varargs
fixed more places where the signature had changed
Merge pull request #735 from davecheney/198-state-infer-endpoints-takes-varargs

state: make InferEndpoints take varargs

Most of the callers, usually the tests, were having to construct a []string, just to satisfy the contract of the method.

Make the method take a varargs ...string which is consistent with other Relation methods.