2.0.3 won't mount if stunnel doesn't exist

[tomalok]

Presumably with the introduction of efs-proxy, we no longer need stunnel on the system.

However, there's still a check for the existence of stunnel which will prevent mounting...

/var/log/amazon/efs # service nfsmount start
 * Caching service dependencies ...                                                                                           [ ok ]
 * Starting NFS sm-notify ...                                                                                                 [ ok ]
 * Mounting NFS filesystems ...
Failed to locate stunnel in /sbin:/usr/sbin:/usr/local/sbin:/root/bin:/usr/local/bin:/usr/bin:/bin - Please install it following the instructions at: https://docs.aws.amazon.com/efs/latest/ug/using-amazon-efs-utils.html#upgrading-stunnel
mount: mounting fs-0344f17a77af334e1 on /mnt/efs/s0 failed: No such device                                                    [ !! ]
 * ERROR: nfsmount failed to start

2024-08-06 00:18:49 UTC - INFO - version=2.0.3 options={'tls': None, 'accesspoint': 'fsap-xxxxxxxxxxxxxxxx', 'fsc': None}
2024-08-06 00:18:49 UTC - INFO - binding 20810
2024-08-06 00:18:49 UTC - ERROR - Command '['which', 'stunnel']' returned non-zero exit status 1.```

Adding the stunnel package back on the system unblocks the mount, and I've confirmed that stunnel itself isn't actually being used...

/var/log/amazon/efs # ps wwaxu|grep stunnel
root      6575  0.0  2.5  19064  9600 ?        Ssl  Aug05   0:00 /usr/bin/efs-proxy /var/run/efs/stunnel-config.fs-0344f17a77af334e1.mnt.efs.s0.20420 --tls

This error probably should be changed to a warning for efs-utils 2.x or only fail with error if the mount options absolutely require stunnel.