Stop trying to create the 'ironic' user twice

After 09bd7c8fd5a6 landed, a conflict between lib/ironic and
extras.d/50-ironic.sh was exposed, breaking Ironic's check and gate
tests.

This resolves that conflict by only creating the 'ironic' user once.

Change-Id: Ic41517f0977c84a82f92f58565aaee6b5cc7eb3e
Merge "Stop trying to create the 'ironic' user twice"