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

Added checks in update method that perform the following:
    1) Look to see if 'tenant' key is set, if it is not set we're good
    2) If 'tenant' key is set, make sure its value is not "" (blank)
    3) If 'tenant' key is set, make sure its value is not "  " (whitespace)

Fixes rm1003
Merge pull request #236 from rackspace-titan/blank_tenant_update

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