Currently if a pod is being scheduled with no meta.RolesKey label
attached to it, per convention the first configured mesos (framework)
role is being used.
This is quite limiting and also lets e2e tests fail. This commit
introduces a new configuration option "--mesos-default-pod-roles" defaulting to
"*" which defines the default pod roles in case the meta.RolesKey pod
label is missing.
- The TASK_ERROR task status was introduced with Mesos 0.21 and is actually used since 0.22.
It was not handled at all before this patch, leaving errored task in the registry in phase
"Pending". This will lead to task status updates from the Mesos Master on reconciliation with empty
slaveId fields, leading to scheduler crashes eventually.
- Handle terminal task with empty slaveId.
The slave id can be empty for TASK_ERROR.
The modified code path does not use the slaveId.
