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
Merge "Store/restore Server from a dict with credentials"