Non conflicting AWS_ENV_REGION variable

[adudek]

AWS_REGION is widely used across multiple aws tools, yet due to parameter-store nature we need to use different region quite often.
Instead of var cycling or sub-shelling (reducing env+stack traceability) its better to use dedicated variable (AWS_ENV_REGION)

proposal (logical solution):
AWS_ENV_REGION || AWS_REGION || current_region(user-data) || null