Description of problem:
instackenv.json file starts the set of nodes with following structure:
{
  nodes: [...]
}
which is not accepted by parser.


Version-Release number of selected component (if applicable):
Master


How reproducible:
100 %


Steps to Reproduce:
1. Install RDO-Manager via instack using virtual environment
2. Try to register nodes with unified CLI using instackenv.json file


Actual results:
Parsing error.


Expected results:
Any file containing list named "nodes" will be accepted.


Additional info:
See https://repos.fedorapeople.org/repos/openstack-m/instack-undercloud/html/baremetal-setup.html for example structure of the file.

"openstack baremetal import --json" should support both file structures:
* simple list starting with `[]`
* any list starting with `nodes: []`