glance client.py doesn't support client cert

[Shinya Tsunematsu]

openstackclient/image/client.py doesn't support client cert, so "openstack server create" failed even if we set both OS_CERT and OS_KEY.

below is a part of error message of "openstack server create" command with --debug option.

curl -g -i -X GET -H 'Accept-Encoding: gzip, deflate' -H 'Accept: */*' -H 'User-Agent: python-glanceclient' -H 'Connection: keep-alive' -H 'X-Auth-Token: {SHA1}XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX' -H 'Content-Type: application/octet-stream' -k --cert None --key None https://api-endpoint.example.com/glance/v2/images/ubuntu-16.04-server-cloudimg-amd64
Starting new HTTPS connection (1): api-endpoint.example.com
/path/to/.pyenv/versions/2.7.9/lib/python2.7/site-packages/requests/packages/urllib3/connectionpool.py:852: InsecureRequestWarning: Unverified HTTPS request is being made. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings InsecureRequestWarning)

we notice that --cert and --key has no attributes.

[Shinya Tsunematsu (tnmt)]

we notice that "openstack image show" also failed, maybe from same reason.