Allow to specify current user in config

[baturin]

Ability to specify user in config file, to get simple way to switch user in development environment, with no need to modify "mw.py", and with no need to configure OAuth.

[codecov-commenter]

Codecov Report

Merging #173 into master will decrease coverage by 0.00%. The diff coverage is 38.46%.

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
- Coverage   76.52%   76.51%   -0.01%     
==========================================
  Files          19       19              
  Lines        3766     3769       +3     
  Branches      471      472       +1     
==========================================
+ Hits         2882     2884       +2     
- Misses        643      644       +1     
  Partials      241      241              

Impacted Files	Coverage Δ
montage/mw.py	78.51% <38.46%> (-0.57%)	:arrow_down:
montage/rdb.py	77.96% <0.00%> (+0.06%)	:arrow_up:

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[mahmoud]

Upon further review, I think we can give this one a shot. su_to is still respected, and this should help with local testing.