dragonfly should read .project8_authentications.json at start

[wcpettus]

bin/dragonfly should handle reading and parsing the .project8_authentications.json file, and pass along the relevant keys to the services:

• this will simplify the priority order of the broker in dripline-python (project8/dripline-python#16)
• this will prevent the authentications file from being read multiple times (again by slack or the postgres interfaces to get their keys)

the keys should not be stored in variables once used, make sure they are only passed as arguments so they aren't retrievable via a get