Added explicit wait to avoid reading default grading value

This should fix this failure: https://jenkins.testeng.edx.org/job/edx-all-tests-auto-master/184/SHARD=1,TEST_SUITE=cms-acceptance/testReport/junit/CMS/Course%20Grading%20_%20User%20can%20set%20a%20grace%20period%20greater%20than%20one%20day/Then_I_see_the_grace_period_is__48_00_/

I haven't been able to reproduce it locally, but I think the issue is that backbone is putting in a default value "00:00" before changing it to the actual value.  When we read the value too soon, the test fails.

Reviewer: @jzoldak