Add new KeyPair class and methods for the key pair management to the NodeDriver
class.
Update existing compute drivers (OpenStack, CloudStack, EC2) which expose key
pair management through the extension methods to expose it through a new
standard key pair management API.

Also update affected code and tests, deprecate existing extension methods and
add some missing tests.

Note: Old and now deprecated methods will work until a next major release.
docs: Update upgrade notes,  add a new section about key pair management
and update existing docs.
Add and implement get_key_pair method.

Also add tests and update documentation.
Update fixture generation script to include key pair management methods in the
resulting fixtures.
docs: Include key pair management supported methods table.
docs: re-generate fixtures.
Merge branch 'promote_key_pair_management_to_base_compute_api' into trunk
docs: re-organize upgrade notes, move non breaking stuff towards the end.
Add badges to the readme.