Host reboot and shutdown backend implementation

This patch includes the functions to enable reboot and shutdown of a
Kimchi host. It also enables the Rest API, and includes POST actions
documentation in docs/API.md. Shutdown and reboot are only allowed
if there are not vms running.

Signed-off-by: Adriano Botega <abotega@linux.vnet.ibm.com>
Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>