Add support for BOINC Universal Docker Application (BUDA).

[davidpanderson]

This lets you run arbitrary Docker applications using a single
BOINC app (and one app version per platform).
The Dockerfile and science executables are in the workunit.

The script tools/submit_buda lets you test this.
Basic tests were successful.

2. Unify the scripts for testing job submission
   The 'test' assimilators (sample_assimilate.py and sample_assimilator.cpp)
   do the same thing: they copy result files to
   <proj_dir>/results/<batch_id>/<wu_name> (if 1 output file)
   <proj_dir>/results/<batch_id>/<wu_name>_i (if >1 output file)
   where <batch_id> is 0 if WU is not in a batch
   ... and they write error code to <wu_name>_error if the WU errored out
   Scripts to submit jobs:
   submit_job
   submit_batch
   submit_buda
   Script to query jobs:
   query_job
   This works for either jobs or batches,
   as long as the app uses one of the above assimilators

3. Add plan class 'docker' to plan_class_spec.xml.sample

Fixes #5896

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.

Project coverage is 10.73%. Comparing base (abfea4c) to head (2d35fd5).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
sched/sample_assimilator.cpp	0.00%	16 Missing ⚠️
sched/script_assimilator.cpp	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5897      +/-   ##
============================================
- Coverage     10.73%   10.73%   -0.01%     
  Complexity     1068     1068              
============================================
  Files           280      280              
  Lines         36598    36604       +6     
  Branches       8482     8484       +2     
============================================
  Hits           3930     3930              
- Misses        32279    32285       +6     
  Partials        389      389              

Files with missing lines	Coverage Δ
sched/script_assimilator.cpp	0.00% <0.00%> (ø)
sched/sample_assimilator.cpp	0.00% <0.00%> (ø)