Add the --prefix option to scope daemon

[michalbiesek]

Related to #1361.

This will allow to use daemon in following environment:

• in k8s scope daemon can be used as a container in separate pod with proper privileges related to Ebpf code:
• privileges
• mountPath /sys/kernel/debug
  This is sufficient to handle snapshot file but to retrieve the files generated by the library from other containers/pods we need to have access to /proc view from host.

There is the comment in the code which describes this issue:

appscope/cli/snapshot/snapshot.go

Lines 195 to 203 in 9c9bb65

	// If process is in a parallel container or a host above (i.e. in an above / parallel namespace) // proc/pid not visible unless hostfs
	// (pid provided must be of the hosts perspective)
	// (requires --privileged flag? or mounted hostfs?)
	// get crash files - use /proc/pid/root or --getfiles
	// get hostname file - use /proc/pid/root or --getfiles
	// generate snapshot - call gensnapshot
	// get process username,environ - from /proc using --getfiles
	// Unsupported at this time.

[michalbiesek]

The second thing besides above is after discussion with @jrcheli is when run scope k8s we should ensure that scope daemon should run in every node - this thing can be done I think via https://kubernetes.io/docs/concepts/workloads/controllers/daemonset/