Nomad should accept multiple consul certificate authorities ca_file or ca_path

When configuring SSL with consul, it is possible to specify a CA path containing multiple CA certificates instead of just a single CA file. it's the [`ca_path`](https://www.consul.io/docs/agent/options.html#ca_path) directive (as opposed to [`ca_file`](https://www.consul.io/docs/agent/options.html#ca_file)).

This can allow graceful renewal of CA certificates. When the first CA is nearing its end of life, a new CA certificate is generated and added to that directory. Then all consul clients and servers should be updated to change their certificates to use a certificate signed with the new CA. because both CA are accepted, it makes no problem.

Nomad however doesn't have the ability, as a consul client, to specify multiple consul CA. For graceful migration, it should be possible to allow that.

Start by locating Nomad's Consul client configuration and the existing ca_file handling in the Go code. Trace how Consul TLS settings are passed to the client, then identify the relevant tests; done means a ca_path containing multiple certificates is accepted alongside the existing ca_file behavior.

Written by the indexing model from the issue text.