Allow to request a particular set of plugins when starting introspection

Allow users to alter or replace set of plugins to be used for a node.
Add new parameter to POST /v1/introspect/<UUID> and client.introspect:
- processing_hooks - comma-separated string (HTTP argument) or list of string (client API) - replace list of processing hooks for the node, use * char to substitute current list of hooks, e.g.:

processing_hooks=plugin1
 runs only one plugin - plugin1
processing_hooks=*,plugin1
 appends plugin1 to the processing chain

The resulting set of plugins to apply should be created and checked on starting introspection and stored in options table in `processing_hooks` field.