Return 409 on creating/importing same name keypair

* Removed the DB look up on creating and importing a keypair.
* Added unique constraint on user_id and name columns of key_pairs
table.

Fixes LP: #1086333.

Change-Id: I2fd4697f9269d2be13bd977e65ba4ca4a27b9ac6
Merge "Return 409 on creating/importing same name keypair"