Add flag USING_VIDEO_PIPELINE to support fallback video

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      1. Description

(https://openedx.atlassian.net/browse/OSPR-1827)

This PR uses the ```fallback_video``` when not using video pipeline by setting the variable ```USING_VIDEO_PIPELINE``` added in ```ios.yml```.

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      1. How to test this PR

Add the flag to ```ios.yml``` and change the values of ```USING_VIDEO_PIPELINE```

```
API_HOST_URL: ''
OAUTH_CLIENT_ID: ''
ENVIRONMENT_DISPLAY_NAME: ''
PLATFORM_NAME: ''
PLATFORM_DESTINATION_NAME: ''
FEEDBACK_EMAIL_ADDRESS: ''
DEBUG: false
ORGANIZATION_CODE: ''
PUSH_NOTIFICATIONS: false
MY_VIDEOS_ENABLED: true
USING_VIDEO_PIPELINE: false
COURSE_ENROLLMENT:
ENABLED: true
TYPE: 'native'
COURSE_SHARING_ENABLED: false
DISCUSSIONS_ENABLED: true
CERTIFICATES_ENABLED: true
USER_PROFILES_ENABLED: true
BADGES_ENABLED: true
REGISTRATION_ENABLED: true
SEGMENT_IO:
SEGMENT_IO_WRITE_KEY: ''
ENABLED: false
```

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      1. Reviewers

• [ ] Code review: @saeedbashir

• [ ] Code review: @salman2013

cc @marcotuts @gsong