Change the sample_type from tuple to string

The Collector can not persistence compute node CPU metrics when set
‘compute_monitors = ComputeDriverCPUMonitor’ in nova.
The root cause of that issue is the counter_type is the 'tuple',
In actually the type of the counter_type should be String,
so it can not persistence to DB success, please refer to the SQL,
the counter_type value is [u'cumulative'] that is a 'tuple'.

Change-Id: I8d1218f813d081e1031c331c59a17c227c9a8958
Closes-Bug: #1295004
Merge "Change the sample_type from tuple to string"