Set missing @odata.context

[kbarbounakis]

Describe the bug
Service Root URL response does not include odata.context annotation

like other OData services

Additional context

Annotation odata.context
http://docs.oasis-open.org/odata/odata-json-format/v4.0/errata03/os/odata-json-format-v4.0-errata03-os-complete.html#_Toc453766628