DC/OS application group support

[siggy]

linkerd-viz does not properly aggregate metrics from DC/OS applications deployed as part of groups.

For example, an app named my-group/webapp yields metrics like this:

rt:outgoing:dst:id:_:io_l5d_marathon:my_group:webapp:requests 11

the last two metrics_relabel steps defined at:
https://github.com/BuoyantIO/linkerd-viz/blob/master/dcos/prometheus-dcos.yml#L32

.... cause the metric to be rewritten as:

linkerd:incoming:webapp:requests{instance="10.0.2.164",job="linkerd",service="my_group"}

...when the expected metric should be:

linkerd:incoming:requests{instance="10.0.2.164",job="linkerd",service="my_group/webapp"}

[klingerf]

Fixed by #19.