travis: Don't duplicate Docker invocation
travis: Don't duplicate Docker invocation

All variable parts are taken from the environment, so we
can exploit inheritance and avoid duplication. This will
become more important as we start building on additional
platforms.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

zippy2/libvirt,jfehlig/libvirt,andreabolognani/libvirt,libvirt/libvirt,eskultety/libvirt,zippy2/libvirt,andreabolognani/libvirt,crobinso/libvirt,eskultety/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,eskultety/libvirt,jfehlig/libvirt,crobinso/libvirt,crobinso/libvirt,andreabolognani/libvirt,jardasgit/libvirt,olafhering/libvirt,jfehlig/libvirt,nertpinx/libvirt,libvirt/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,eskultety/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jardasgit/libvirt,olafhering/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,libvirt/libvirt,eskultety/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,libvirt/libvirt,jardasgit/libvirt,nertpinx/libvirt,crobinso/libvirt