networkdb: Properly format memberlist logs
networkdb: Properly format memberlist logs

Right now, items logged by memberlist end up as a complete log line
embedded inside another log line, like the following:

    Nov 22 16:34:16 hostname dockerd: time="2016-11-22T16:34:16.802103258-08:00" level=info msg="2016/11/22 16:34:16 [INFO] memberlist: Marking xyz-1d1ec2dfa053 as failed, suspect timeout reached\n"

This has two time and date stamps, and an escaped newline inside the
"msg" field of the outer log message.

To fix this, define a custom logger that only prints the message itself.
Capture this message in logWriter, strip off the log level (added
directly by memberlist), and route to the appropriate logrus method.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

flavio/docker,moul/docker,thaJeztah/docker,nawawi/docker,thaJeztah/docker,cpuguy83/docker,jimjag/docker,djs55/docker,konstruktoid/docker-upstream,jameszhan/docker,cpuguy83/docker,nalind/docker,thaJeztah/docker,nawawi/docker,jimjag/docker,trebonian/docker,gnailuy/docker,djs55/docker,docker/docker,moul/docker,jimjag/docker,cpuguy83/docker,Mokolea/docker,bearstech/moby,bearstech/moby,thaJeztah/docker,cpuguy83/docker,nawawi/docker,mahak/docker,bearstech/moby,nalind/docker,AkihiroSuda/docker,moul/docker,Ramzec/docker,jthelin/docker,yosifkit/docker,djs55/docker,docker/docker,paralin/docker,moby/moby,AkihiroSuda/docker,docker/docker,Mokolea/docker,nawawi/docker,flavio/docker,moby/moby,nalind/docker,flavio/docker,trebonian/docker,Ramzec/docker,moby/moby,Mokolea/docker,paralin/docker,konstruktoid/docker-upstream,gnailuy/docker,nalind/docker,flavio/docker,konstruktoid/docker-upstream,jameszhan/docker,vdemeester/docker,moby/moby,jameszhan/docker,moul/docker,mahak/docker,konstruktoid/docker-upstream,vdemeester/docker,AkihiroSuda/docker,vdemeester/docker,jimjag/docker,Ramzec/docker,jthelin/docker,paralin/docker,Mokolea/docker,gnailuy/docker,paralin/docker,Ramzec/docker,mahak/docker,mahak/docker,gnailuy/docker,yosifkit/docker,yosifkit/docker,yosifkit/docker,trebonian/docker,trebonian/docker,AkihiroSuda/docker,bearstech/moby