Added support for replicated/global mode for service & stack

[subfuzion]

Closes #255

Verification

$ make test

$ amp service create --name pinger appcelerator/pinger
# verify service created with 1/1 replicas
$ amp service rm pinger

$ amp service create --replicas 2 --mode replicated --name pinger appcelerator/pinger
# verify service created with 2/2 replicas
$ amp service rm pinger

$ amp service create --mode global --name pinger appcelerator/pinger
# verify service created as global
$ amp service rm pinger

$ amp service create --replicas 4 --mode global --name pinger appcelerator/pinger
# verify error: replicas can only be used with replicated mode

[generalhenry]

the verify error resulted in a 2016/09/26 19:31:39 grpc: addrConn.resetTransport failed to create client transport: connection error: desc = "transport: dial tcp: context canceled"; Reconnecting to {"localhost:50101" <nil>}

Which is no good, but I think it's pretty much unrelated to this PR

[subfuzion]

@generalhenry ruh-roh. Let's see if @chrisccoy can duplicate. Thanks.

[chrisccoy]

I also am seeing the issue reported by @generalhenry, however, it is not every time. I believe it is the problem with the /etc/hosts entry. If the amplifier service does not come up clean, then I see the 2016/09/26 20:17:20 grpc: addrConn.resetTransport failed to create client transport: issue, but I have had several clean starts and witnessed the test suite works as expected.

I believe that in certain instances, the swarm is restarting amplifier resulting in a bad entry in the hosts file. Not sure of an elegant approach to fix the issue, but this does appear to be pre-existing and the PR was successful.

[subfuzion]

@chrisccoy Thanks for the analysis. We definitely need to investigate further. 👍