add node_selector_enabled variable

[rptaylor]

This is an alternative to #29 @y2kenny

Default behaviour before: there would be no nodeselectors applied because nfd.enabled is false by default. If you set nfd.enabled=true, then you would get both an installation of NFD, and application of nodeselectors on the daemonsets.

Default behaviour after: there would be no nodeselectors applied because node_selector_enabled is false by default. Installation of NFD and application of nodeselectors can be controlled separately and independently.

Anyone who previously set nfd.enabled=true and got both a NFD installation and nodeselectors will now need to also set node_selector_enabled=true to continue applying the same nodeselector as before.

It's a bit weird but seems like a less disruptive way to un-overload the nfd.enabled variable.

[rptaylor]

Partially related to #19

@julian3xl @boniek83 have a look if you're still interested.