Standardised the validate logic found azure mounts

[storey247]

• Moved validation logic for azure mounts to central place
• Added validation function to the adls gen 2 mount
• Added unit tests for the validation logic

This PR fixes issue #94

[codecov-commenter]

Codecov Report

Merging #95 into master will increase coverage by 0.52%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage   47.26%   47.79%   +0.52%     
==========================================
  Files          53       53              
  Lines        6838     6884      +46     
==========================================
+ Hits         3232     3290      +58     
+ Misses       3553     3543      -10     
+ Partials       53       51       -2     

Flag	Coverage Δ
#client	76.45% <ø> (ø)
#provider	39.51% <100.00%> (+0.75%)	⬆️

Impacted Files	Coverage Δ
databricks/mounts.go	1.80% <100.00%> (+1.80%)	⬆️
...ricks/resource_databricks_azure_adls_gen1_mount.go	37.33% <100.00%> (+15.45%)	⬆️
...ricks/resource_databricks_azure_adls_gen2_mount.go	37.10% <100.00%> (+0.39%)	⬆️
databricks/resource_databricks_azure_blob_mount.go	36.23% <100.00%> (+14.56%)	⬆️

[stikkireddy]

Hey looks great! thanks again @storey247 much appreciated 😃