BB-3517 Change build success/fail email subject

Created by: arjunsinghy96

Changes the build success/fail email subject as requested by Ned here

JIRA Tickets: BB-3517

Discussions: https://discuss.openedx.org/t/how-do-we-build-continuous-integration-for-the-supported-installation/3065/30?u=0x29a

Merge deadline: None

Testing Instructions:

1. Checkout to this PR
2. ssh to the vagrant box
3. run make test.one instance.tests.models.test_openedx_instance.OpenEdXInstanceTestCase and check all the tests pass
4. In file instance/models/mixins/utilities.py check that the email subjects are according to this comment

Reviewers:

• @farhaanbukhsh
• @0x29a