kill useless Config import
don't forget logger parameter
add required import
add no-op finish() method

it is called from the task whatever implementation is used.
get rid of most thread availability tests

Threads dependency is now constrained to netdiscovery and netinventory
specific engine implementations.
oops
no more support for --local <file>

This was removed in favor of generalisation of brokers to handle
results.
adjust documentation
re-add VERSION variable
more consistency in version tests
Merge remote-tracking branch 'forge/master'