BB-3592 Send email on successful instance update

Created by: arjunsinghy96

Sends email to the BetaTestApplication user on successful instance redeployment of changes made by user.

JIRA Tickets : BB-3592

Merge deadline: None Testing instructions:

1. Checkout to this PR
2. ssh to the vagrant box
3. run make test.one 'registration.tests.test_api.InstanceDeploymentAPITestCase.test_send_email_on_application_redeployment'and check all the tests pass

Reviewers

• @farhaanbukhsh