Accept local CMC capacity value of 0

[alex-chew]

Follow-up to awslabs/aws-encryption-sdk-specification#166.

Current implementation:

aws-encryption-sdk-python/src/aws_encryption_sdk/caches/local.py

Lines 43 to 44 in c961999

	if self.capacity < 1:
	raise ValueError("LocalCryptoMaterialsCache capacity cannot be less than 1")

[farleyb-amazon]

We have longer term plans to rework our cache APIs. Resolving.