LU-16502 lutf: fix bugs in bash scripts

LU-16502 lutf: fix bugs in bash scripts

Addressed some issues I found when running
LUTF. The "rm" fails without a file to remove.
A Lustre wiki led me to source the script, but this
will log out of the current shell. Adding a warning
against doing this.

Also, fix shellcheck errors and a few warnings in
LUTF related scripts. Added bash shebangs, since
shellcheck requires these to lint the scripts.

Lustre-change: https://review.whamcloud.com/49728
Lustre-commit: 3cd0bb6968fc3432b7d1641bf0489e4e4b1e809f

Test-Parameters: trivial env=SANITY_EXCEPT="101j" testlist=sanity
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I501d58d25bfcd6564755485b9a1afa2277848b96
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54519
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>