Fix a typo.
LIBCLOUD-509: Add extra dictionary to the NodeSize object within the base compute API. This will allow additional properties specific to providers to be stored within the object. The GCE driver was updated and the addition of the # of CPUs was added into the CloudStack driver.

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
image -> size and updated docstrings

Signed-off-by: Tomaz Muraus <tomaz@apache.org>