bots: Declare ubuntu-1804 image (#10336)
bots: Declare ubuntu-1804 image (#10336)

Ubuntu 18.10 just got released, so ubuntu-stable will move from 18.04 to
18.10. 18.04 is an LTS which we want to support now instead of 16.04.

Closes #10336
cockpituous/cockpit,andreasn/cockpit,deryni/cockpit,cockpituous/cockpit,martinpitt/cockpit,deryni/cockpit,garrett/cockpit,martinpitt/cockpit,andreasn/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,garrett/cockpit,moraleslazaro/cockpit,andreasn/cockpit,cockpituous/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,deryni/cockpit,martinpitt/cockpit,martinpitt/cockpit,andreasn/cockpit,mvollmer/cockpit,garrett/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,andreasn/cockpit,deryni/cockpit,garrett/cockpit,andreasn/cockpit,andreasn/cockpit,cockpituous/cockpit,deryni/cockpit,deryni/cockpit,cockpit-project/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,mvollmer/cockpit