Add integration test for vms --vitals

Signed-off-by: Phan Le <ple@pivotallabs.com>
Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
Use ConcreteSigar in StatsCollector

* Switch to FakeSigar in tests

Signed-off-by: Kris Hicks <krishicks@gmail.com>
Use Sigar stats access methods

Signed-off-by: Kris Hicks <krishicks@gmail.com>