Provide error when engine version is unsupported

[dongluochen]

Also fix node count in swarm info. Fix #1675.
Signed-off-by: Dong Chen dongluo.chen@docker.com

[dongluochen]

Here is test result.

Nodes: 2
 : 192.168.56.203:4444
  └ Status: Pending
  └ Containers: 0
  └ Reserved CPUs: 0 / 0
  └ Reserved Memory: 0 B / 0 B
  └ Labels: 
  └ Error: engine 192.168.56.203:4444 is running an unsupported version of Docker Engine. Please upgrade to at least 1.6.0
  └ UpdatedAt: 2016-01-25T23:57:00Z
...

[vieux]

LGTM

[abronan]

LGTM

[dongluochen]

Thanks @vieux @abronan!