Keep log files produced by augur tree

[huddlej]

Removes all lines where tree builders remove log files during the clean up step
such that users can inspect logs after any run (successful or
unsuccessful). Updates functional test for the Zika build to confirm that the
log file exists as expected after running.

This commit also updates the functional tests for the Zika build such they
always run the local augur executable (i.e., the one relative to the test
directory). This change is most helpful when running tests on a local copy of
the repository where a global augur install might already exist.

This issue was raised on the Nextstrain discussion board.

[codecov]

Codecov Report

Merging #572 into master will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #572      +/-   ##
==========================================
+ Coverage   23.70%   23.72%   +0.01%     
==========================================
  Files          32       32              
  Lines        5159     5155       -4     
  Branches     1300     1298       -2     
==========================================
  Hits         1223     1223              
+ Misses       3885     3881       -4     
  Partials       51       51              

Impacted Files	Coverage Δ
augur/tree.py	9.94% <0.00%> (+0.20%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d4e5081...7ce221f. Read the comment docs.

[jameshadfield]

The log-related changes look fine to me :)
I've never used bin/augur since I run an editable pip install so can't comment there but this seems sensible

[rneher]

looks good! Thanks!