Existing images update name, visibility etc

Dropped default values of min_disk and min_ram parameters because
it interferes with the update mechanism and glance uses those values
anyway [1], [2].

If the image is already present and visibility param is defined we
should check its visibility and correct it if needed. Added tests
to verify that both is_public and visibility can change the image.

If both name and id are specified for the image, we might want to
update the image name. This rely on fact that id pram is checked first.
Added rename tests to verify this.

For some reason if image object is used for the image update, 409
error produced and exception trown, but the change is in place. So
instead of image object, update query rely on the image.id

[1] https://github.com/openstack/glance/blob/75051dd5a2cdeef2919ba2062714bde7502416ba/glance/db/simple/api.py#L226
[2] https://github.com/openstack/glance/blob/75051dd5a2cdeef2919ba2062714bde7502416ba/glance/domain/__init__.py#L125

Change-Id: I9ca6b78bec96b69e6946b65796f12314a1ec6ab4