Update environmental variable references and code cleanup
Update environmental variable references and code cleanup

The pgBackRest container now recognizes a more standard environmental
variable for working with pgBackRest and as such, the code needs to
be updated to recognize that.

Additionally, this presented an opportunity to clean up the code
a bit so it will be easier to amend in the future.

Issue: [ch9871]
CrunchyData/crunchy-containers,CrunchyData/crunchy-containers,CrunchyData/crunchy-containers