qa/suites: do not use cephadm.exec when it is not needed

Convert a uses of `cephadm.exec` to basic `exec`. When templates (with
the jinja2-style replacement pattern `{{}}`) are not present the
cephadm.exec and exec are essentially the same.

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

qa/suites/nvmeof/thrash/gateway-initiator-setup/10-subsys-90-namespace-no_huge_pages.yaml

@@ -13,7 +13,7 @@ tasks:
 - cephadm.wait_for_service:
     service: nvmeof.mypool.mygroup0
 
-- cephadm.exec:
+- exec:
     host.a:
       - ceph orch ls nvmeof --export > /tmp/nvmeof-orig.yaml
       - cp /tmp/nvmeof-orig.yaml /tmp/nvmeof-no-huge-page.yaml