Resolve "Replace instances of os.environ.get() where appropriate"

Closes #211 (closed)

This MR standardizes usage of os.environ.get() and changes os.environ.get() to os.environ[] when the environment variable is not optional or it does not have a default value.

Successful pipeline runs:

• https://repo1.dso.mil/dsop/opensource/pipeline-test-project/kubectl/-/jobs/1768268 (all vars set)
• https://repo1.dso.mil/dsop/opensource/pipeline-test-project/kubectl/-/jobs/1768296 (unset var in repo map)