dcb: set all Priority Group options at the same time (rh #799241)
First, lldpad doesn't support disabling priority groups (e:0) without specifying a complete priority group config (which wouldn't be used anyway, since you're turning it off!). While this bug is being fixed upstream, we'll just ignore errors turning off PG, since if you're using DCB on an interface, you probably want to use it all the time. Second, lldpad really wants all PG options on the same configuration line, not split apart, because it validates the complete package of options before applying them, regardless of whether or not they are given in the same command. Since NM was just emitting all the options in separate dcbtool invocations anyway, just combine them all into a single invocation.
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