lease_create now raises InvalidDate exception

The ValueError exception raised by datetime.datetime.strptime was
replaced by a new Climate exception: InvalidDate
Added a new unit test to check that an exception is raised when a
lease_create is called with an start date before the actual date.

Change-Id: Iac95373c59bd154fcc162f0665edf0ea8487a8b1
Closes-Bug: #1252704