LU-13423 tests: cleanup_netns correctly set result

LU-13423 tests: cleanup_netns correctly set result

The existence test for 'test1pl' should not result in
cleanup_netns returning failure to the caller.

A slightly more terse if/else can be used to ensure the
caller is notified of failure only in the case of
test1pl not being deleted.

Lustre-change: https://review.whamcloud.com/38157
Lustre-commit: 410b655c71849e5a26251f7c187b19ed8f504bd7

Test-Parameters: trivial
HPE-bug-id: LUS-8713

Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I85dee20ec0f0ccd0be17597431fcedda9469d9da
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44203
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>