cubicweb-pyramid #11376632 generate secret strings for auth and session cookies [in-progress]

These strings should be generated upon instance creation. This would allow to start an instance directly after creation without having to fill these configuration options.

We could, for instance, use paste.script.util.secret.secret_string, which apparently pyramid already depends on.

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patch[ccplugin] add gen-pyramid-config command so one can generate default values for pyramid.ini and pyramid-debug.ini (closes #11376632) [reviewed]Generate secret string for auth and session cookies [pending-review]