Set KRB5 as default clone URL when Kerberos is enabled and user is logged in

_Originally opened at !363 by @cernvcs._

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In GitLab instances where Kerberos authentication (gitlab-org/gitlab-ee!6) is enabled, this protocol becomes the preferred one.

This merge request extends the functionality provided in gitlab-org/gitlab-ee!70 where Kerberos url is displayed alongside the SSH and HTTP urls when Kerberos authentication is enabled.

The implementation proposed here makes Kerberos the default clone procotol when Kerberos is enabled. When a logged in user visits the main page for a project or creates a new one, the first url offered is the Kerberos one.

This merge request also aligns the [implementation](https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/helpers/button_helper.rb) and [usage](https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/views/shared/_clone_panel.html.haml) of the 3 clone buttons, adds additional tests and updates some others to consider all the possible scenarios.

See merge request !372