Fix #3116 - [BUG] DEB is using RPM default file path

[etgraylog]

Description

In this PR is a code-change to fix #3116 - add the Debian location of default env vars to the opensearch.service template.

The fix is to take advantage of the dash (-) prefix of SystemD's EnvironmentFile= option. We can add multiple options, as if the file does not exist, it will not be read and no error or warning message is logged per SystemD documentation:

"The argument passed should be an absolute filename or wildcard expression, optionally prefixed with "-", which indicates that if the file does not exist, it will not be read and no error or warning message is logged. This option may be specified more than once in which case all specified files are read. If the empty string is assigned to this option, the list of file to read is reset, all prior assignments have no effect."

This approach is also used currently for the opensearch-dashboards.service as well.

Issues Resolved

#3116

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[peterzhuamazon]

Thanks @etgraylog for the fix here will take a look.

cc: @bbarani

[peterzhuamazon]

Hi @etgraylog,

We will accept this fix but it wont take effect until the file is being permanently moved to /etc/default/ on debian.
We think moving the file location would be a broken change if user starts to make tweaks on defaults and expect the file to be in /etc/sysconfig/.

We will make the permanently file path change in 3.x.

Thanks.

cc: @bbarani