Prevent caller from specifying id during KeyPair.create()

The patch is to remove id when it was set before creating a key pair
in objects, and check if the key pair exists before it is created.

Related to blueprint icehouse-objects.

Change-Id: I07609500e1135f54929265e06193c48e1bb083db
Merge "Prevent caller from specifying id during KeyPair.create()"