analyzer hierarchy support

[hunchback]

as illustrated in https://github.com/skydive-project/skydive/pull/2336 I wanted to construct a hierarchy of analyzers:

• broker - the central hub which aggregated all information
• east/west - which should only report upwards

but, alas this option is not supported and as a result:

• when accessing the east analyzer you see also broker and west
• when accessing the broker you sometimes see a duplication of east or west (as this information may echo between the analyzers)

the desired change is for a subordinated analyzer to be able to stop accepting updated from the top-level analyzer - so for example if east is configure currently:

analyzers: 
- broker:30082

etcd:
  embedded: false
  servers:
  - http://broker:30079

the change would require to add:

analyzer:
    # mode: in our case relevant for broker
    # mode: readwrite
    mode: read

agent:
    # mode: in our case relevant for east and west
    # mode: readwrite
    mode: write

which would bring the result that:

• only broker shows the aggregate view including 3 clusters
• east and west will only show their single cluster

[lebauce]

@hunchback Addressed by https://github.com/skydive-project/skydive/pull/2369