When doing remote execution, it was sometimes failing with the following
exception: OSError: [Errno 10] No child processes.
The problem is when using libev default loop, the latest register a handler for
SIGCHLD, it causes a problem when Popen.communicate() tried to do a wait_pid,
the syscall is skipped because of the signal. Then when wait_pid is called
again, it failed because the process don't exist anymore.

When the node is disconnected from the cc-server, it keeps it internal state
until the connection pass, then in case the same role is returned by the
cc-server, it just reregister its objects and tags, else it remove the previous
main plugin and recreate a new one corresponding to the adequate role.

Support for registering/unregistering sub object to the cc-server.
Support for tag lifecycle handling (libev), was previously managed by plugins.

Plus error handling.

Refactored plugins.
Updated hypervisor libvirt event handling.
Async rpc handling.
Minor renaming for libvirt event callback.
Remove uneeded method sub_tags (in hypervisor Handler).

Authentication and host handler.

Initialization, re-authentication...