Provide option to supply GCP CMEK crypto_key_path as a single configuration item

[nj1973]

The current configuration for CMEKs in GCP relies on access to the KMS API in order to obtain crypto_key_path from GOOGLE_KMS_KEY_RING_LOCATION, GOOGLE_KMS_KEY_RING_NAME and GOOGLE_KMS_KEY_NAME. This can sometimes prove problematic. An alternative approach is to allow crypto_key_path to be provided in a single variable, e.g. GOOGLE_KMS_KEY_PATH, ideally allowing us to remove the former variables.