:openstack_auth_url should not require trailing /tokens

[plribeiro3000]

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Rubiojr's gist is the first Google hit for "ruby fog openstack". Notice how the parameters for Fog::Compute.new there are mostly direct equivalents of the OS_* environment variables of the Unified CLI, except for OS_AUTH_URL_FOG. That is because the common OS_AUTH_URL is the "identity API endpoint" which, as quoted by the /tokens API output itself, is of the form scheme://host:5000/2.0. OS_AUTH_URL_FOG currently needs to be ${OS_AUTH_URL}/tokens.

I think :openstack_auth_url should be compatible with the unified CLI and not require the trailing /tokens.

A related issue is that the code distinguishing v1 authentication from v2 requires a trailing / after the 2.0.

Original issue created by @mvidner at fog/fog#1210.

[plribeiro3000]

cc/ @mvidner @geemus @its-not-a-bug-its-a-feature @mwhagedorn @krames @ushrivatsa @mdarby