gravel: clean up systemdisk

[jecluis]

This is preparatory clean up work for a bigger cleanup and rework of how we deploy the system. In part to account for an inevitable change in how we install the system, but mostly (and more pressing) to better handle error states.

Signed-off-by: Joao Eduardo Luis \joao@suse.com

[codecov-commenter]

Codecov Report

Merging #752 (b74985d) into main (6a18377) will decrease coverage by 0.59%. The diff coverage is 50.00%.

:exclamation: Current head b74985d differs from pull request most recent head 498e7f5. Consider uploading reports for the commit 498e7f5 to get more accurate results

@@            Coverage Diff             @@
##             main     #752      +/-   ##
==========================================
- Coverage   70.13%   69.53%   -0.60%     
==========================================
  Files          30       30              
  Lines        3171     3217      +46     
==========================================
+ Hits         2224     2237      +13     
- Misses        947      980      +33     

Impacted Files	Coverage Δ
src/gravel/controllers/nodes/systemdisk.py	79.80% <48.43%> (-15.11%)	:arrow_down:
src/gravel/controllers/nodes/deployment.py	66.84% <100.00%> (ø)

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[jecluis]

The function now takes a list, but the code is still relying on shlex because I think that's less error prone.