Separate feature discovery and creation of feature labels in the system
source.
Also, change the implementation of the nodeName custom rule to utilize
the FeatureSource interface of the system source.
Also, add minimalist unit test.
There are cases when the only available metadata for discovering
features is the node's name. The "nodename" rule extends the custom
source and matches when the node's name matches one of the given
nodename regexp patterns.
It is also possible now to set an optional "value" on custom rules,
which overrides the default "true" label value in case the rule matches.
In order to allow more dynamic configurations without having to modify
the complete worker configuration, custom rules are additionally read
from a "custom.d" directory now. Typically that directory will be filled
by mounting one or more ConfigMaps.
Signed-off-by: Marc Sluiter <msluiter@redhat.com>
