add unit attribute to statistics

statistics currently aren't associated with a unit it just
provides sum, average, etc... values without a unit context

Fixes: Bug#1204458
Change-Id: Ib92a6a2ab5899a0b269424e94468a73b6559deda
Removed unused self.counters in storage test class

Fixes bug #1210278

The list variable self.counters currently doesn't do anything useful in the
storage test class StatisticsTest, so the variable should be dropped.

Change-Id: Ife1d423d93ab4e2feb43efd54b3e5e7a415b3340
Remove unused abstract class definitions

Fixes bug #1210281

Removed abstract classes definitions from DBTestBase and EventTestBase classes
in storage driver tests since they are not used.  Neither of these classes has
defined any abstract methods, so the abstract class definitions are unnecessary.

Also removed import statement for abstract base classes since there are now no
abstract class definitions.

Change-Id: I0b5b487631d780ba2ab087dce2ef340f126f8a69
Merge "Removed unused self.counters in storage test class"
Merge "Remove unused abstract class definitions"
Merge "add unit attribute to statistics"