Add a MockAzureClient which can simulate azure failures

[muhammadhamzasajjad]

This is similar to #1281.

• Adds a config option to use the MockAzureClient instead of the RealAzureClient
• If a MockAzureClient is created, one can simulate failures by passing the appropriate symbol name
• Creates tests to ensure that MockAzureClient works properly for read, write, update, delete and list functions.

To do: Test exceptions thrown by MockAzureClient. This will be done in the next PR where we normalize Azure exception.

Checklist

Checklist for code changes...

• Have you updated the relevant docstrings, documentation and copyright notice?
• Is this contribution tested against all ArcticDB's features?
• Do all exceptions introduced raise appropriate error messages?
• Are API changes highlighted in the PR description?
• Is the PR labelled as enhancement or bug so it appears in autogenerated release notes?

[poodlewars]

Looks good! Please squash the commits in to one before merging