Ssh key management for softlayer

This pull requests add ssh key management for the SoftLayer driver.

Add methods for:
list_key_pairs()
get_key_pair()
import_key_pair_from_string()
delete_key_pair()
create_key_pair()

It also adds a new property to create node "ex_keyname" which is the name of the key to be associated to the new node.

Added tests for all methods and modified the test_create_node() test to use a sshkey as well.
Key pair management is now part of the base compute API so you should use the existing `libcloud.compute.base.KeyPair` class.