Adds ssh key management to Softlayer compute driver

Adds the following methods for the Softlayer driver:
 - list_key_pairs()
 - get_key_pair()
 - import_key_pair_from_string()
 - delete_key_pair()
 - create_key_pair() (only if Pycripto is installed)

Adds tests for all new methods.
Modifies the test_create_node() to use an ssh key.
Adds a new property on the Softlayer node "ex_keyname"which is the name of the key to be associated to the new node.