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.
We should probably also allow user to specify key size and default to `4096` bits just to be on the safe side.

For example: `def create_key_pair(self, name, ex_size=4096)`