PRE_CLEAR_INIT_GROOVY_D: Add in ability to pre-clear the init.groovy.d folder

[ac-hibbert]

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Currently you can populate the init.groovy.d folder with scripts that you want to run and append .override if you require to override current scripts of the same name. However you cannot override the whole directory, this may be necessary if you wish to remove scripts, change names of scripts, change run order etc. This fix allows the user to pre-clear the init.groovy.d folder prior to the files being copied into place, meaning the files you copy over to $REF/init.groovy.d will be the only ones present

[MarkEWaite]

Seems fine, anyone else got any thoughts on it? @MarkEWaite ?

It seems like this Is introducing a specific implementation of a more general concept. I think the general concept is that a subdirectory in JENKINS_HOME is to be completely managed by the contents copied from the base image.

The .override technique allows a single file to always replace the same file in the JENKINS_HOME directory. Should we extend the concept to an entire directory?