Allow multiple labels per builder node.

Allows for a builder to be specified by many projects.

If many projects want to run on this node they either have to specify a label which must be shared amongst builder recipes or the last builder recipe will overwrite any prior labels that are specified.

This pull-request fixes this by concatenating all labels that are specified by many builder recipes during a single chef run together and writing them onto the node rather than the current behavior of writing the value of the last cookbook onto the node.