Add Flannel troubleshooting
Add Flannel troubleshooting

Add section on Flannel to troubleshooting guide.

Flannel provides the overlay network and is the default
network driver for Kubernetes COE.  It is also an optional
network driver for Swarm COE.  Failure in Flannel would
mean that the containers cannot communicate with each other.

This section covers common failures, how to verify configuration,
how to check for correct operation, logs, known limitations.

If you have encountered other scenarios or have additional tips,
please comment or add your own patch.

Partially implements:  blueprint magnum-troubleshooting-guide
Change-Id: Icfc61d532a3381f3b70d36d8e765f0b6548e0c3f

ArchiFleKs/magnum,openstack/magnum,ArchiFleKs/magnum,jay-lau/magnum,openstack/magnum