Duplicated lease name should return HTTP 409

Added new manager exception 'LeaseNameAlreadyExists',
wraps db exception ClimateDBDuplicateEntry occures during lease creation
and returns in the API as:
HTTP 409 The lease with name: %(name)s already exists

Current implementation returns HTTP 500 Internal server error,
which is not user friendly.

Change-Id: I3ead720eda9b08980e3ee7c5cf878354b5455877
Closes-Bug: #1302584\
Merge "Duplicated lease name should return HTTP 409"