cubicweb #2800884 add spec file to newcube skeleton [validation pending]

Just like they get a debian packaging template, new cubes should get a template spec file.

prioritynormal
typeenhancement
done in3.17.0
load2.000
load left0.000
closed by#ada2f065f279 skeleton: add RPM spec template (closes #2800884)
patchskeleton: add RPM spec template [applied]