central database config changes

is now configured from [CENTRALAGENT] section
default name changed to "central_db.sqlite3"
move central agent code from plugin to core class

- the code for the actual central agent now belongs to the
  TNArchipelCentralAgent class instead of the "centralagent" plugin
- the "centralagent" plugin is for exclusive use of the hypervisor, to
  communicate with central agent.
move central agent code from plugin to core class

- the code for the actual central agent now belongs to the
  TNArchipelCentralAgent class instead of the "centralagent" plugin
- the "archipel-agent-centraldb" plugin is for exclusive use of the hypervisor, to
  communicate with central agent.
add archipel-agent-centraldb plugin

the hypervisor plugin which communicates with central agent