Cleanup: Add common "create keypair" method

There are many "create keypair" methods in scenario tests, their
behaviors are almost the same. They create keypairs with random names.
This patch adds common "create keypair" method and uses it instead
of each implementation.

Change-Id: I9355b9e19604b6c2e648b3473d1de47fe19fa9ea
Merge "Cleanup: Add common "create keypair" method"