Update CHANGES.
Issue LIBCLOUD-469: Add Reserved Instance support to EC2 driver.

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Modify EC2ResercedNode to call parent constructor and drop unused arguments from
the constructor.
Also pass "size" attribute to EC2ReservedNode constructor.
Move test from NimbusTests to EC2Tests class.
list_reserved_nodes -> ex_list_reserved_nodes, update docstrings.
Update CHANGES.