plugins: fix system.kernel_cmdline_arguments() call

In commit 2c8e71ff82d we introduced new codebase for autoinstalls.
However, we should use system.kernel_cmdline_arguments() instead of
system.kernel_cmdline_args(). This patch fix the call.

Change-Id: I72f9ef678623dd6ee0c835d160151981283255dc
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>