ledger-tool: Minor cleanup on accountsdb config argument parsing

[steviez]

Summary of Changes

Really minor change here; made the change as part of something else but decided to break it out to avoid cluttering the other PR

• Avoid parsing the accounts_index_path twice when it is present
• Reorder the parsing so that the "associated" items appear next to each other. Namely, put all of the parts of AccountsIndexConfig next to each other instead of having the test_partitioned_epoch_rewards smack in the middle of it

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b814497) 81.8% compared to head (549beab) 81.8%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #34671   +/-   ##
=======================================
  Coverage    81.8%    81.8%           
=======================================
  Files         824      824           
  Lines      222394   222394           
=======================================
+ Hits       181957   182054   +97     
+ Misses      40437    40340   -97     

[gregcusack]

lgtm. apologies for the slow review 😑