LIBCLOUD-392: Add support for keypairs management in the OpenStack driver.

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Updated argument names in the ex_import_keypair and ex_import_keypair_from_string methods to be consistent with other drivers.

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Use with context manager so we don't leak open files.
Update CHANGES.