Adopt OSB API to support adapt credentials

[kramerul]

Issue Template

Step 1: Is this issue for here?

Yes

Step 2: Describe the problem (very short text description):

We would like to use parts of cloudfoundry/servicebroker#586 for the istio-broker

Step 3: Describe the problem (steps):

n/a

Observed Results:

n/a

Expected Results:

n/a

Relevant Code Snippet:

See #180