Change boolean environment variables to case insensitive

[prodrigestivill]

Proposal to change all boolean environment variables to case insensitive.
Currently this affects this variables:

• BACKUP_ON_START
• POSTGRES_CLUSTER

Comment:

It is apparently case sensitive changing it to BACKUP_ON_START: "TRUE" did the trick for me, but I'd suggest making it lowercase or even 1 instead of true for convenience.

Originally proposed by @TiraelSedai in #159 (comment)