rest: allow to use timespan to define archive policy

Users can now use a timespan to specify an archive policy, which makes
it easier to define what one might actually want to retrieve as users:

POST /v1/archive_policy
=>  {"name": "foobar",
    "definition": [{"timespan": "1 day", "granularity": 5}]}

<= {"name": "foobar",
    "definition": [{"timespan": "1 day", "granularity": 5, "points": 17280}]}

Change-Id: I121a7c307c93b2ffc8e65f681b62d61a70b7cbd7