Store/restore Server from a dict with credentials

The patch adds two methods to Server class to get a dict with
credentials from instance of its class and restore an instance from
credentials. LxcProvider requires reconstruction of the Server objects
for destruction of resources.

blueprint db-resource-track

Change-Id: I2f9ca4a1c49477d665ba02ff0736c21a7557e12a
LxcProvider track resources

The patch introduce a resource tracking in LxcProvider.

blueprint db-resource-track

Change-Id: I4bb0cb9d9af5203a6de147903c944fc883d3bd8f