Fix default file backup config

[t-bast]

The file-backup section should be inside eclair, otherwise startup fails.

[pm47]

Embarrassing. Here is a micro post mortem:

• the bug wasn't caught by the integration tests because we explicitly disable db backups for them (using the correct config path)
• the bug wasn't caught by the StartupSpec test either because the config isn't handled by NodeParams.

[codecov-commenter]

Codecov Report

Merging #1857 (d2dad2e) into master (f52c3dd) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1857      +/-   ##
==========================================
- Coverage   88.96%   88.93%   -0.03%     
==========================================
  Files         150      150              
  Lines       11250    11250              
  Branches      457      457              
==========================================
- Hits        10008    10005       -3     
- Misses       1242     1245       +3     

Impacted Files	Coverage Δ
...clair/blockchain/bitcoind/rpc/BatchingClient.scala	86.36% <0.00%> (-13.64%)	⬇️
.../acinq/eclair/blockchain/bitcoind/ZmqWatcher.scala	96.09% <0.00%> (-0.79%)	⬇️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	86.27% <0.00%> (+0.08%)	⬆️