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.

Closes #354

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