install managers group by default

[lferran]

fixes #778

[codecov-io]

Codecov Report

Merging #960 into master will decrease coverage by 5.2%.
The diff coverage is 68.2%.

@@           Coverage Diff            @@
##           master    #960     +/-   ##
========================================
- Coverage    94.6%   89.4%   -5.1%     
========================================
  Files         315     315             
  Lines       27876   27437    -439     
========================================
- Hits        26356   24517   -1839     
- Misses       1520    2920   +1400     

Impacted Files	Coverage Δ
guillotina/tests/dbusers/test_manage_groups.py	80.6% <22.3%> (-19.4%)	⬇️
guillotina/auth/groups.py	87.5% <100.0%> (-1.3%)	⬇️
guillotina/contrib/dbusers/install.py	76.0% <100.0%> (+1.0%)	⬆️
guillotina/tests/dbusers/test_setup.py	100.0% <100.0%> (ø)
guillotina/tests/test_postgres.py	10.0% <0.0%> (-90.0%)	⬇️
guillotina/tests/test_cockroach.py	17.7% <0.0%> (-77.9%)	⬇️
guillotina/db/storages/utils.py	34.8% <0.0%> (-65.2%)	⬇️
guillotina/db/storages/pg.py	26.5% <0.0%> (-62.4%)	⬇️
guillotina/db/storages/vacuum.py	47.0% <0.0%> (-53.0%)	⬇️
guillotina/db/storages/cockroach.py	27.9% <0.0%> (-48.1%)	⬇️
... and 119 more