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TLS parameters can be passed through the following environment
variables:
* OPENSTACK_CA_FILE - CA file path
* OPENSTACK_CERT_FILE - cert file path
* OPENSTACK_KEY_FILE - key file path. Either both or none of cert/key
  must be specified
* OPENSTACK_TLS_SERVER_NAME - (optional) expected CN of the server
  certificate if differs from domain in the URL
* OPENSTACK_TLS_INSECURE - if set to yes|true|1 disables validation of
  the server certificate

Code that loads tls.Config from environment variables was extracted
into dedicated package so that it could be reused by different providers