Keypair API test cleanup

This is a cleanup in preparation for adding more
test coverage around keypairs. This patch does the
following:

* Moves keypair tests into their own file

* Breaks tests for methods into logically separate
  classes

* Shares key name validation and quota checking
  tests between `create` and `import_key`

* As part of above, add missing tests for
  `import_key` that were present for
  `create`

Change-Id: I8d8fdf9a1f9f11e1015ebbdfde1c8e77f5f421a8
Merge "Keypair API test cleanup"