Add volume test

Test for preserving partitions from core device.
Test for preserving partitions at core device, which were made on exported object,
after stopping cache.
Also added two fields to 'Core' class which allows it to use 'create_partitions' function.

Signed-off-by: Slawomir Jankowski <slawomir.jankowski@intel.com>

test/functional/api/cas/core.py

@@ -33,6 +33,8 @@ class Core(Device):
         if core_info["exp_obj"] != "-":
             Device.__init__(self, core_info["exp_obj"])
         self.cache_id = cache_id
+        self.partitions = []
+        self.block_size = None
 
     def __get_core_info(self):
         output = TestRun.executor.run(