General: Benevolent context label collector

[iLLiCiTiT]

Brief description

Context label collector is more benevolent to available data.

Description

Collector does not require currentFile to be filled and use just host name in that case. If context data contains contextLabel the plugin just set it as label of context and skip rest of logic, this was added to have ability define custom context label and skip the plugin logic. Result is that standalone publisher and tray publisher don't have to be explicitly named in the plugin.

Pyblish pype does not force to have "label" in instance.data and context.data.

Testing notes:

1. Context label should be collected as did