remove set -u and reserved variable USERNAME

[maozza]

fix unbound variable error, the script call USERNAME and PASSWORD before setting it. USERNAME is reserved in Ubuntu 18.04

This pull request makes the following changes:

• remove set -u due to error .helm/plugins/helm-nexus-push.git/push.sh: line 104: PASSWORD: unbound variable
• change reserved variable USERNAME (on ubuntu linux USERNAME is my os username)

(If there are changes to user behavior in general, please make sure to update the docs, as well)

It relates to the following issue #s:

• Fixes #X

[sonatypecla[bot]]

Thanks for the contribution! Unfortunately we can't verify if the committer(s), maoz [email protected], signed the CLA because they have not associated their commits with their GitHub user. Please follow these instructions to associate your commits with your GitHub user. Then sign the Sonatype Contributor License Agreement and this Pull Request will be revalidated.

[dtshepherd]

@maozza See PR #1. We'll likely work on getting that one merged.