EX-8059 tests: hot-pools to work in a single node

EX-8059 tests: hot-pools to work in a single node

hot-pools.sh doesn't start on a single node setup because it tries
to find lpurge on $(all_osts_nodes) is empty. make single node
setup an exclusion.

Test-Parameters: trivial
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I6520ff7bef92d62fd39f4c9347462d046844ecea
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51925
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>