RecipeParser: Support comments in tasks

There is the bug in the way the <task> tags were processed. The
processor didn't ignore comments and considered them to be invalid
commands.

This commit fixes that.

Reported-by: Ondrej Lichtner <olichtne@redhat.com>
Signed-off-by: Radek Pazdera <rpazdera@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>