Passing a blank tenant when creating a schedule does not throw an error

Added checks for both whether the key is set in the fixture, as well as if the key exists. There are two unit tests I wrote in test_schedules called test_create_blank_tenant and test_create_no_tenant. Incorporated changes from previous pull request comments.

Fixes rm1003
Merge pull request #231 from rackspace-titan/tenant_blank

Passing a blank tenant when creating a schedule does not throw an error