Give the SDN permission to create Events

[danwinship]

#16745 made the SDN log an event on startup, but the system:sdn-reader group doesn't actually have permission to create Events; it was just stealing it from system:node. Now that the SDN is running separately, it needs to explicitly have that permission.

events is forbidden: User "system:serviceaccount:openshift-sdn:sdn" cannot create events in the namespace "default"

Current version of this PR just adds event create permission to system:sdn-reader, which is probably wrong I guess since "create" != "read". So do we need to create a new system:sdn group for the SDN pod to run under? And deprecate sdn-reader since it would no longer serve any purpose?

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1581538

[knobunc]

/lgtm

[openshift-ci-robot]

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