merge node-mixin and node-observ-lib

[v-zhuravlev]

node_exporter mixin sync and directories merge:

• finally add missing USE dashboards:
  
  
  
• add rows to node-observ-lib. It is easier to create new dashboards on the fly by using rows instead of single panels.
• add AIX dashboards (sync from upstream) (Add AIX support prometheus/node_exporter#3136)
• Add missing ".json" to all dashboards
• sync and update some alerts
  • from upstream (https://github.com/grafana/node_exporter/pull/31/files#diff-f4042050be686df1df36fa998eb23965d887f05d9c219840e7fc9fc67279ac7f)
  • Add NodeHasRebooted(info) alert
  • Add NodeProcessesCountIsHigh alerts

The rest of diff is just about moving files around.