bots: Download fedora-26 image during tests-invoke
bots: Download fedora-26 image during tests-invoke

Lets download the fedora-26 image during tests-invoke. It's used
by tests like the networking tests to run a DHCP server. Doing
it during the tests results in slow downloads due to wasteful
parallel downloads.

Closes #7331
martinpitt/cockpit,stefwalter/cockpit,petervo/cockpit,cockpituous/cockpit,stefwalter/cockpit,petervo/cockpit,larsu/cockpit,andreasn/cockpit,michalskrivanek/cockpit,martinpitt/cockpit,stefwalter/cockpit,mareklibra/cockpit,xhad/cockpit,SotolitoLabs/cockpit,larskarlitski/cockpit,mvollmer/cockpit,stefwalter/cockpit,Scribery/cockpit,mareklibra/cockpit,larsu/cockpit,michalskrivanek/cockpit,harishanand95/cockpit,harishanand95/cockpit,petervo/cockpit,moraleslazaro/cockpit,harishanand95/cockpit,petervo/cockpit,larsu/cockpit,larsu/cockpit,SotolitoLabs/cockpit,larsu/cockpit,garrett/cockpit,Scribery/cockpit,michalskrivanek/cockpit,moraleslazaro/cockpit,deryni/cockpit,harishanand95/cockpit,cockpituous/cockpit,deryni/cockpit,SotolitoLabs/cockpit,garrett/cockpit,andreasn/cockpit,SotolitoLabs/cockpit,xhad/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,xhad/cockpit,Scribery/cockpit,mareklibra/cockpit,andreasn/cockpit,deryni/cockpit,andreasn/cockpit,larskarlitski/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,michalskrivanek/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,garrett/cockpit,Scribery/cockpit,martinpitt/cockpit,Scribery/cockpit,larskarlitski/cockpit,larskarlitski/cockpit,cockpituous/cockpit,mareklibra/cockpit,xhad/cockpit,SotolitoLabs/cockpit,harishanand95/cockpit,larskarlitski/cockpit,deryni/cockpit,larskarlitski/cockpit,deryni/cockpit,mareklibra/cockpit,garrett/cockpit,mvollmer/cockpit,larsu/cockpit,harishanand95/cockpit,petervo/cockpit,moraleslazaro/cockpit,andreasn/cockpit,martinpitt/cockpit,deryni/cockpit,mvollmer/cockpit,xhad/cockpit,xhad/cockpit,moraleslazaro/cockpit,andreasn/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,michalskrivanek/cockpit,andreasn/cockpit,mareklibra/cockpit,cockpituous/cockpit,michalskrivanek/cockpit,stefwalter/cockpit,mareklibra/cockpit,cockpituous/cockpit,mvollmer/cockpit,Scribery/cockpit,harishanand95/cockpit,stefwalter/cockpit,larskarlitski/cockpit,xhad/cockpit,moraleslazaro/cockpit,Scribery/cockpit,petervo/cockpit,martinpitt/cockpit,stefwalter/cockpit,michalskrivanek/cockpit,cockpit-project/cockpit,garrett/cockpit,petervo/cockpit,deryni/cockpit,cockpit-project/cockpit