Customisable token headers

[c0c0n3]

What we will need for the future (not now), is a description of how to add/change the headers that will go into the signature process. But this is step 2

@gboege I reckon if it's just about choosing the header names to use, that's a no-brainer. It can be done with Istio operator config when mapping incoming requests to Mixer attributes as in:

https://github.com/orchestracities/boost/blob/dev/deployment/sample_operator_cfg.yaml#L27

[c0c0n3]

For now, change "ids-dth" on line 27

https://github.com/orchestracities/boost/blob/dev/deployment/sample_operator_cfg.yaml#L27

to "header" as in:

ids_dth_header:  request.headers["header"]

[c0c0n3]

Implemented by PR #12---see note there.