[bugfix] Fix crash of local scheduler with flexible allocation

[vkarak]

This introduces a an API for a Node, which the Job talks to for determining
whether the node is available or not. Different backends implement that
differently.

Also made private the SlurmNode class and the create_nodes() function.

Fixes #1087.

[codecov-io]

Codecov Report

Merging #1088 into master will decrease coverage by <.01%.
The diff coverage is 89.28%.

@@            Coverage Diff            @@
##           master   #1088      +/-   ##
=========================================
- Coverage   91.71%   91.7%   -0.01%     
=========================================
  Files          81      81              
  Lines       11110   11121      +11     
=========================================
+ Hits        10190   10199       +9     
- Misses        920     922       +2

Impacted Files	Coverage Δ
unittests/test_schedulers.py	96.76% <100%> (+0.02%)	⬆️
reframe/core/schedulers/local.py	100% <100%> (ø)	⬆️
reframe/core/schedulers/__init__.py	95.68% <100%> (+0.06%)	⬆️
reframe/core/schedulers/slurm.py	55% <50%> (ø)	⬆️
reframe/core/config.py	82.9% <0%> (-1.71%)	⬇️

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