Add support for multitenant context brokers (FIWARE-Service header)

[justb4]

This request comes from StackOverflow

Basically, add a new preference for indicating what tenant is to be used for connecting to the context broker (if the preference is empty, then the FIWARE-Service header is not used).

See similar issue and fix here: wirecloud-fiware/ngsi-browser-widget#1

[manchandavishal]

fixed in #2