Simplify initialization of Empire core

[ejholmes]

Closes #688
Closes #606
Fixes #624
Fixes #525

This moves all of the configuration into cmd/empire. empire.New goes from 116 lines to just 20 lines and this makes it really simple to use a different scheduler implementation, since it's now just:

e := empire.New(db, empire.Options{})
e.Scheduler = someScheduler

This also makes it almost impossible to improperly configure empire when starting up, because it won't use fake implementations of interfaces. Everything is much more explicit now.

ECS configuration is now also logged from cmd/empire on start, making it easier to debug configuration issues:

server_1   | 2015/12/13 04:27:13 Using ECS backend with the following configuration:
server_1   | 2015/12/13 04:27:13   Cluster: empire-e01a8fac-Cluster-1L3NSOKE3TM4G
server_1   | 2015/12/13 04:27:13   ServiceRole: empire-e01a8fac-ServiceRole-1NX8XQ7IWC3LS
server_1   | 2015/12/13 04:27:13   InternalSecurityGroupID: sg-e7387381
server_1   | 2015/12/13 04:27:13   ExternalSecurityGroupID: sg-1938737f
server_1   | 2015/12/13 04:27:13   InternalSubnetIDs: [subnet-bb01c4cd subnet-c85f4091]
server_1   | 2015/12/13 04:27:13   ExternalSubnetIDs: [subnet-bb01c4cd subnet-c85f4091]
server_1   | 2015/12/13 04:27:13   ZoneID: Z3GZSDLUUUW245
server_1   | 2015/12/13 04:27:13 Streaming logs are disabled
server_1   | 2015/12/13 04:27:13 Starting on port 8080

[ejholmes]

This is fixed in #687

[phobologic]

👍