Set network "from" / fix trace logic

[cgewecke]

• Allow configuring default sender account in Buidler plugin #552 Fixes bug (in HH) where from address is not inherited from the network config (plus tests)
• Fixes tracing logic - the tracking was not bound to the trace step

[codecov-io]

Codecov Report

Merging #562 (d230def) into hardhat (dba6e0c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           hardhat     #562   +/-   ##
========================================
  Coverage    99.05%   99.05%           
========================================
  Files           22       22           
  Lines          950      953    +3     
========================================
+ Hits           941      944    +3     
  Misses           9        9           

Impacted Files	Coverage Δ
lib/api.js	100.00% <100.00%> (ø)
lib/collector.js	100.00% <100.00%> (ø)
plugins/hardhat.plugin.js	97.70% <100.00%> (ø)
plugins/resources/nomiclabs.utils.js	100.00% <100.00%> (ø)

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