Poll for no servers in autoscaling test

Heat treats server in DELETED state as deleted,
which leaves for a (rare) possibility to still find
a non-empty list of servers in the end of this test.

Poll for half a minute for no servers in the server list,
and use detailed server list API to have a better picture of
what was server status if this check does nevertheless finds
any servers and fails.

Related-Issue: PRODX-54144
Change-Id: Ia2d859843017e331d4ad0a2f97d0be5302208dec
1 file changed