Fix issue where file system mount to invalid directory caused plan to fail

[storey247]

• Removed redundant call to dbutils.fs.ls() when reading mounts
• Created interface for CommandExecutor
• Added unit tests for mount logic which use interface to allow mocking of the call to execute db command
• Fixed minor linting issues

Essentially the dbutils.fs.ls call was causing the error to be thrown. This is because it checks that the directory exists. When reading the mounts from the db workspace we don't really care if they are actually valid mounts, just that they exist and if their configuration has changed.

Fixes issue #105

[nfx]

Looks good

[codecov-commenter]

Codecov Report

Merging #106 into master will increase coverage by 1.11%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   47.11%   48.22%   +1.11%     
==========================================
  Files          56       56              
  Lines        7138     7135       -3     
==========================================
+ Hits         3363     3441      +78     
+ Misses       3724     3634      -90     
- Partials       51       60       +9     

Flag	Coverage Δ
#client	76.45% <ø> (ø)
#provider	40.44% <33.33%> (+1.41%)	⬆️

Impacted Files	Coverage Δ
client/service/commands.go	93.93% <ø> (ø)
databricks/resource_databricks_aws_s3_mount.go	33.33% <0.00%> (ø)
...ricks/resource_databricks_azure_adls_gen1_mount.go	37.33% <0.00%> (ø)
...ricks/resource_databricks_azure_adls_gen2_mount.go	37.10% <0.00%> (ø)
databricks/resource_databricks_azure_blob_mount.go	36.23% <0.00%> (ø)
databricks/mounts.go	30.29% <45.45%> (+28.48%)	⬆️

[stikkireddy]

LGTM now thx for the update