fix(hoststack): hardcode DUT1 for nginx

fix(hoststack): hardcode DUT1 for nginx

Previously, the Set up LDP or VCL Nginx on DUT node keyword
was wrongly documenting its "dut" argument as type string,
while using the value as a node dict instead.

As most of the code already used direct dut1 variables,
this patch eliminates the dut argument altogether
and puts DUT1 into the keyword name.

Change-Id: Ie29afc175b98c1b1cb554438da9178738662d108
Signed-off-by: Vratko Polak <vrpolak@cisco.com>