Uneeded config warnings when using keys in env vars ([pkcs7] both public_key and public_key_env_var specified, using public_key)

[rbelnap]

When specifying keys as env vars, I receive warning messages on every operation:

[pkcs7] both public_key and public_key_env_var specified, using public_key
[pkcs7] both private_key and private_key_env_var specified, using private_key

This looks to be due to

hiera-eyaml/lib/hiera/backend/eyaml/encryptors/pkcs7.rb

Line 47 in 5ece2fe

warn "both public_key and public_key_env_var specified, using public_key"

and

hiera-eyaml/lib/hiera/backend/eyaml/encryptors/pkcs7.rb

Line 73 in 5ece2fe

warn "both public_key and public_key_env_var specified, using public_key"

The issue seems to be that because public_key has a default, it is always defined. I cannot find any way to unset it, so this warning displays any time env vars are used for keys. The warning is also misleading, since it does use the env var as expected.