travis: Deduplicate build instructions
travis: Deduplicate build instructions

All information, except for osx_image image, is identical between
the two jobs so we can avoid repeating it.

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

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