Implement certificate-related options

[kshpytsya]

Implement equivalents of the following nomad job dispatch options:

  -ca-cert=<path>
    Path to a PEM encoded CA cert file to use to verify the
    Nomad server SSL certificate.  Overrides the NOMAD_CACERT
    environment variable if set.

  -ca-path=<path>
    Path to a directory of PEM encoded CA cert files to verify
    the Nomad server SSL certificate. If both -ca-cert and
    -ca-path are specified, -ca-cert is used. Overrides the
    NOMAD_CAPATH environment variable if set.

  -client-cert=<path>
    Path to a PEM encoded client certificate for TLS authentication
    to the Nomad server. Must also specify -client-key. Overrides
    the NOMAD_CLIENT_CERT environment variable if set.

  -client-key=<path>
    Path to an unencrypted PEM encoded private key matching the
    client certificate from -client-cert. Overrides the
    NOMAD_CLIENT_KEY environment variable if set.

  -tls-skip-verify
    Do not verify TLS certificate. This is highly not recommended. Verification
    will also be skipped if NOMAD_SKIP_VERIFY is set.