Use names containing "non-existing" as non-existing resource names

In test_list_servers_by_non_existing_flavor, 1234 is used as
non_existing_flavor, but it is not strange if someone happens
to use 1234 as flavor id in his environment.

Besides, we can use names containing 'non-existing' instead
of a hard-coded strange name for non-existing image/server/flavor
names, which will make code looks more clear.

Change-Id: I97e5a8723a77b5e5626ec20b439a5e6ce729d511
1 file changed