This allows the same environment variables to be read, parsed, and used
from any API client as was previously handled in the CLI. The CLI now
uses the API environment variable reading capability, then overrides any
values from command line flags, if necessary.
Fixes#618
Bugfix: When walking a given CA path, the walk gives both files and
directories to the function. However, both were being passed in to be
read as certificates, with the result that "." (the given directory for
the CA path) would cause an error. This fixes that problem by simply
checking whether the given path in the walk is a directory or a file.
Feature enhancement: VAULT_CACERT, VAULT_CAPATH, and VAULT_INSECURE now
perform as expected.
