Enhance QPU failover by penalizing failed solvers

[randomir]

Fixes #477, with a caveat -- user-supplied (or default num qubits-based) solver preference is ignored on trigger_failover(). We override order_by in order to de-prioritize failed solvers.

In summary, this is a quickfix for #477, and a nicer fix should respect the lower-level solver preference, but penalize failed solvers. This seems hard to implement in dwave-system without a lot of code duplication from dwave-cloud-client. One possible, and pretty general, approach is to apply order_by transformations sequentially (in the cloud-client), one per config level, instead of higher-level setting overridding a lower-level setting.

[codecov-commenter]

Codecov Report

Merging #478 (d02e37d) into master (44de5dc) will decrease coverage by 4.15%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
- Coverage   90.29%   86.14%   -4.16%     
==========================================
  Files          24       24              
  Lines        1669     1674       +5     
==========================================
- Hits         1507     1442      -65     
- Misses        162      232      +70     

Impacted Files	Coverage Δ
dwave/system/samplers/dwave_sampler.py	83.72% <100.00%> (-1.31%)	:arrow_down:
dwave/system/coupling_groups.py	12.50% <0.00%> (-81.25%)	:arrow_down:
dwave/system/samplers/leap_hybrid_sampler.py	61.42% <0.00%> (-14.29%)	:arrow_down:
dwave/system/samplers/clique.py	73.85% <0.00%> (-13.73%)	:arrow_down:
dwave/system/composites/embedding.py	96.57% <0.00%> (-1.15%)	:arrow_down:
dwave/system/testing.py	95.23% <0.00%> (-0.80%)	:arrow_down:

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[randomir]

I'll merge this for now as is, and try to generalize it a bit in follow-up PR.