core: Introduce boot time field in VDS

Added boot time as long with seconds since epoch to VDS db record, VDS struct
and REST API.

Change-Id: I0ec679a4ce5ddfbc1735db380feab69eb3798ae6
Bug-Url: https://bugzilla.redhat.com/1070348
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>