test: Add DEFAULT_IMAGE constant to testinfra
test: Add DEFAULT_IMAGE constant to testinfra

This is in preparation for #3205

Closes #3208
Reviewed-by: Marius Vollmer <bb11c6e942c5cc765df5f04da146b3198127a7a3@redhat.com>

petervo/cockpit,SotolitoLabs/cockpit,Scribery/cockpit,Scribery/cockpit,stefwalter/cockpit,xhad/cockpit,mvollmer/cockpit,FireDrunk/cockpit,arilivigni/cockpit,mareklibra/cockpit,martinpitt/cockpit,martinpitt/cockpit,xhad/cockpit,arilivigni/cockpit,dperpeet/cockpit,michalskrivanek/cockpit,sgallagher/cockpit,michalskrivanek/cockpit,xhad/cockpit,moraleslazaro/cockpit,larskarlitski/cockpit,larsu/cockpit,sgallagher/cockpit,Scribery/cockpit,petervo/cockpit,larskarlitski/cockpit,mvollmer/cockpit,garrett/cockpit,michalskrivanek/cockpit,garrett/cockpit,michalskrivanek/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,michalskrivanek/cockpit,cockpit-project/cockpit,michalskrivanek/cockpit,SotolitoLabs/cockpit,harishanand95/cockpit,martinpitt/cockpit,larskarlitski/cockpit,vanloswang/cockpit,martinpitt/cockpit,vanloswang/cockpit,petervo/cockpit,larskarlitski/cockpit,harishanand95/cockpit,xhad/cockpit,mareklibra/cockpit,moraleslazaro/cockpit,andreasn/cockpit,andreasn/cockpit,dperpeet/cockpit,stefwalter/cockpit,petervo/cockpit,larsu/cockpit,harishanand95/cockpit,mareklibra/cockpit,arilivigni/cockpit,garrett/cockpit,petervo/cockpit,larskarlitski/cockpit,petervo/cockpit,vanloswang/cockpit,harishanand95/cockpit,FireDrunk/cockpit,cockpit-project/cockpit,mvollmer/cockpit,vanloswang/cockpit,andreasn/cockpit,dperpeet/cockpit,andreasn/cockpit,larsu/cockpit,arilivigni/cockpit,harishanand95/cockpit,jscotka/cockpit,matobet/cockpit,dperpeet/cockpit,deryni/cockpit,mareklibra/cockpit,mareklibra/cockpit,vanloswang/cockpit,xhad/cockpit,cockpit-project/cockpit,jscotka/cockpit,cockpituous/cockpit,petervo/cockpit,cockpituous/cockpit,dperpeet/cockpit,Scribery/cockpit,harishanand95/cockpit,stefwalter/cockpit,stefwalter/cockpit,larskarlitski/cockpit,jscotka/cockpit,matobet/cockpit,xhad/cockpit,larsu/cockpit,matobet/cockpit,stefwalter/cockpit,andreasn/cockpit,arilivigni/cockpit,stefwalter/cockpit,deryni/cockpit,larskarlitski/cockpit,FireDrunk/cockpit,deryni/cockpit,arilivigni/cockpit,stefwalter/cockpit,larsu/cockpit,mareklibra/cockpit,harishanand95/cockpit,Scribery/cockpit,dperpeet/cockpit,SotolitoLabs/cockpit,vanloswang/cockpit,moraleslazaro/cockpit,andreasn/cockpit,mvollmer/cockpit,jscotka/cockpit,sgallagher/cockpit,xhad/cockpit,FireDrunk/cockpit,SotolitoLabs/cockpit,michalskrivanek/cockpit,arilivigni/cockpit,deryni/cockpit,martinpitt/cockpit,dperpeet/cockpit,moraleslazaro/cockpit,matobet/cockpit,SotolitoLabs/cockpit,FireDrunk/cockpit,garrett/cockpit,jscotka/cockpit,cockpituous/cockpit,vanloswang/cockpit,mareklibra/cockpit,Scribery/cockpit,cockpituous/cockpit,sgallagher/cockpit,sgallagher/cockpit,Scribery/cockpit,sgallagher/cockpit,mvollmer/cockpit,andreasn/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,larsu/cockpit,deryni/cockpit,moraleslazaro/cockpit,garrett/cockpit,sgallagher/cockpit,SotolitoLabs/cockpit,cockpituous/cockpit,matobet/cockpit,matobet/cockpit,moraleslazaro/cockpit,jscotka/cockpit,deryni/cockpit