pithos: Perform size checks during uploads

Verify the consistency between the declared size
and the object's hashmap during uploads/object updates.
pithos: Fix test

The specific test challenges the object upload
by providing the object's hashmap instead of actual data
while not all the respective blocks are previously
uploaded to the storage (409 Conflict status).
However, this test will fail if all the blocks
are stored to the backend
by preceding uploads / test executions.

This commit changes the test in order to upload random data
and, hence, it reduces the possibility of block collisions.