- package cluster: use context.Context as first function param in every
exported function
- package cmd: pass cmd.Context() to calls to package cluster exported
functions
- imageVolume couldn't be determined before in some cases, because it
tried to extract the name from labels on the masterlb which doesn't have
them.
- now we're only trying get the label from master/worker nodes and this
also adds a failover solution
- also, we ignore non-worker/master nodes upon image import
