Use os.getenv's default values

[42B]

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.

Short description of what this PR does:

• Use os.getenv's default values (this also conforms to Python's preference of EAFP over LBYL)

krampus/krampus.py

Lines 55 to 58 in 738f006

	if os.getenv("DEFAULT_REGION"):
	region = os.getenv("DEFAULT_REGION")
	else:
	region = "us-east-1"

>>> os.getenv("DEFAULT_REGION", "us-east-1")
'us-east-1'

krampus/krampus.py

Lines 67 to 70 in 738f006

	if os.getenv("ARN_WHITELIST"):
	whitelist = os.getenv("ARN_WHITELIST")
	else:
	whitelist = None

>>> os.getenv("ARN_WHITELIST") is None
True

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