Customizable init container image

[ntriller-plaid]

Motivation

The init container image is hard coded in the template currently. This PR makes the image customizable so it can be pulled from a different location such as a private registry.

Backwards compatibility

• There are no new required values.
• The image is the same as before by default except that the image is fully qualified now, e.g. alpine:3.9 before and docker.io/library/alpine:3.9 after.

Tests

Added some helm unit tests:

helm unittest .

### Chart [ athens-proxy ] .

 PASS   tests/init_container_image_test.yaml
 PASS   tests/pdb_test.yaml

Charts:      1 passed, 1 total
Test Suites: 2 passed, 2 total
Tests:       5 passed, 5 total
Snapshot:    0 passed, 0 total
Time:        10.602792ms

[DrPsychick]

LGTM, thank you for your contribution!