check_snmp: Handle SNMPv3 noAuthNoPriv properly

The SNMPv3 noAuthNoPriv security level, somewhat unintuitively, requires
a security name to be passed along together with the request. Check_snmp
previously did not do this, causing snmpget to throw an error:

"External command error: No log handling enabled - turning on stderr
logging
snmpget: No securityName specified"

This patch fixes the issue by always providing the security name when
noAuthNoPriv is specified.

See also:
https:://bugs.op5.com/view.php?id=8385.

Signed-off-by: Anton Lofgren <alofgren@op5.com>