Allow policy.json file to be specified

The policy.json file was removed in e5beee63c16ceb3499aaee513d368104684f0412
this will allow you to specify an attribute to point to a location of a
policy.json file to be used.

By default this is nil and the packaged policy.json file will be used,
but if required this can be overridden.

Change-Id: Ic3a1b0c507be627acbee1c74302997fa66a72e4b
Implements: blueprint remote-policy