Fix the check for the existence of the `tls.crt` file on the remote machine in `check_nsfs_tls_cert_setup`

[sagihirshfeld]

The previous check would always fail due to multiple mistakes in the command:

1. not using sudo
2. -d ("is directory) instead of -e ("does this file/directory exist")
3. The use of '/' at the end of the string that is supposed to be the path of a file and not a directory

This would cause the check to always fail and rerun the TLS certification setup, which is both a waste of time and a potential cause of errors - rerunning the TLS certification setup invalidates the previous certificate, which in turn could cause unexpected SSL errors .

Here are the passing logs of the existing tests: https://privatebin.corp.redhat.com/?44159f8535cd5486#CZLJ9aHpLQ8tS8iBh6h5SZmbvDCCsgHEc8hptprTCBJk