Support for Docker checks with Consul

[vgeshel]

Adds support for Docker checks as described in Consul docs. This runs the check command inside the container.

I've tested this with Consul 0.6 (note that consul interprets the check command's exit code 0 as success, 1 as warning, 2 as critical):

host$ docker run -it --rm -e SERVICE_NAME=test1 -e SERVICE_CHECK_DOCKER_SCRIPT="test -f /tmp/x || exit 2" -e SERVICE_CHECK_DOCKER_SHELL=/bin/sh -p 10011:1234 alpine sh

host$ consul-cli health-service test1
...
      {
        "Node": "ip-10-234-180-74.ec2.internal",
        "CheckID": "service:ip-10-234-180-74.ec2.internal:grave_noyce:1234",
        "Name": "Service 'test1' check",
        "Status": "critical",
        "Notes": "",
        "Output": "",
        "ServiceID": "ip-10-234-180-74.ec2.internal:grave_noyce:1234",
        "ServiceName": "test1"
      }
...

container# touch /tmp/x

host$ consul-cli health-service test1
...
  {
        "Node": "ip-10-234-180-74.ec2.internal",
        "CheckID": "service:ip-10-234-180-74.ec2.internal:grave_noyce:1234",
        "Name": "Service 'test1' check",
        "Status": "passing",
        "Notes": "",
        "Output": "",
        "ServiceID": "ip-10-234-180-74.ec2.internal:grave_noyce:1234",
        "ServiceName": "test1"
      }
...

host$ dig test1.service.consul SRV
...
;; ANSWER SECTION:
test1.service.consul.   0   IN  SRV 1 1 10011 ip-10-234-180-74.ec2.internal.node.us-east-1.consul.
...
container# rm /tmp/x

host$ dig test1.service.consul SRV
# nothing

host$ consul-cli health-service test1
...
      {
        "Node": "ip-10-234-180-74.ec2.internal",
        "CheckID": "service:ip-10-234-180-74.ec2.internal:grave_noyce:1234",
        "Name": "Service 'test1' check",
        "Status": "critical",
        "Notes": "",
        "Output": "",
        "ServiceID": "ip-10-234-180-74.ec2.internal:grave_noyce:1234",
        "ServiceName": "test1"
      }
...

[vgeshel]

Just realized that #332 does the same thing. Closing as redundant.