Issue LIBCLOUD-416: Improvements to SoftLayer compute driver

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Fix lint.
Update CHANGES.
[LIBCLOUD-435] Fix a bug in ktucloud driver when diskoffering doesn't exist

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Update CHANGES.
Fix the following auth related things in the OpenStack classes:

- Correctly handle ex_force_auth_token argument
- Correctly cache, re-use and expire auth tokens

Also update affected tests and test fixtures.

Reported by Michael Farrell, part of LIBCLOUD-428.
docs: Add "How do I test if provider credentials are valid?" FAQ entry.
docs: Update contributing page.
Lower timeout in deployment tests. This way the whole test suite finishes
slightly faster.
docs: Include api docs for Node, NodeSize and NodeImage classes.
Add missing docstrings to some classes in the compute API.
Fix format of some return type docstrings (return s -> rtype).
docs: Update wording.
libcloud.compute.base: Add some more missing docstrings, use consistent
docstring format and fix some broken class references.
docs: Also expose NodeAuthSSHKey, NodeAuthPassword and NodeLocation class in the
api docs.
Add new c3 instance types with pricing data to EC2 driver.
Update CHANGES.
Fix invalid docstring.
Fix an issue with the ex_list_keypairs and ex_list_security_groups method
in the CloudStack driver which caused an exception to be thrown if the API
returned no keypairs / security groups.

Reported by Carlos Reategui, part of LIBCLOUD-432.
Fix a typo in the issue number.