Don't read certs from the vault server's local filesystem

The `ca_cert`, `client_cert`, and `client_key` configuration parameters are paths to files. These files are read from the local filesystem the vault server is running on. It would be better if the contents of those files were uploaded and stored in vault not read from the local filesystem.

Start by tracing how the ca_cert, client_cert, and client_key configuration parameters are currently handled and where their contents are passed to Vault. Determine the storage and retrieval path for uploaded certificate contents, then verify that the plugin no longer depends on files on the Vault server's local filesystem.

Written by the indexing model from the issue text.