Port over the 'update' branch fixes into the 7.6.0 release branch

@tcort @Smith I tried to incorporate your changes from the update branch. The major differences between your commits and this one are:

Maintain portability

We need the ability to build the UBI image within Elastic's environment. To that extent, we need the ability to :

• override the NEXUS_SCHEME to use http, but have it default to https
• override the NEXUS_SERVER to use our own server, but have it default to dcar's server

Adopt the new prepare.sh to obfuscate the Nexus creds

I have no idea how the build-scripts/prepare.sh could ever work without adding ARG NEXUS_USERNAME and NEXUS_PASSWORD into the Dockerfile. I added them back in there, which would not print them out during the build and allows the prepare.sh script to not fail.

Fix the issue introduced with the WORKDIR change

The update branch changed the WORKDIR from /usr/local/elasticsearch where the tarball needs to be downloaded to /opt. As a result, the sed command was failing to find files where they should be located. The major change made here was to execute the tar command as such : tar /opt//mypackage.tar.gz instead of tar mypackage.tar.gz

Kindly let me know if this is an issue in your build environment.