manager: kresctl utility

kresctl usage design:

$ kresctl [global_arguments] {command} [command_arguments]

What needs to be done:

• architecture to easily add new commands using argparse
• implement global arguments
  • -s SOCKET to config HTTP API socket
  • -h help, handled automatically by argparse
• implement commands
  • stop command to stop all running daemons
  • config command to {set, delete, get} configuration using HTTP API
  • convert command to convert YAML/JSON config to Lua
  • metrics command to get metrics
  • schema command to get json-schema of the configuration
  • validate command to validate declarative config with the schema
  • reload command to reload config file
• sphinx documentation

Implements #732