[Do Not Merge] Add API for adding external storage account configurations to a web app

[ebencarek]

Redo of #3423 which was already approved, I messed up the commit history when rebasing.

This checklist is used to make sure that common issues in a pull request are addressed. This will expedite the process of getting your pull request merged and avoid extra work on your part to fix issues discovered during the review process.

PR information

• The title of the PR is clear and informative.
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• Swagger files are correctly named (e.g. the api-version in the path should match the api-version in the spec).

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  • Validation tools were run on swagger spec(s) and have all been fixed in this PR.

[ebencarek]

Redo of #3423 which was already approved, I messed up the commit history when rebasing.

[AutorestCI]

Automation for azure-sdk-for-python

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-python#2065

[AutorestCI]

Automation for azure-sdk-for-ruby

Nothing to generate for azure-sdk-for-ruby

[AutorestCI]

Automation for azure-sdk-for-go

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-go#2266

[AutorestCI]

Automation for azure-sdk-for-node

Encountered a Subprocess error: (azure-sdk-for-node)

Command: ['/usr/local/bin/autorest', '/tmp/tmp6d0_0c0z/rest/specification/web/resource-manager/readme.md', '--license-header=MICROSOFT_MIT_NO_VERSION', '--node-sdks-folder=/tmp/tmp6d0_0c0z/sdk', '--nodejs']
Finished with return code -9
and output:

AutoRest code generation utility [version: 2.0.4280; node: v8.11.3]
(C) 2018 Microsoft Corporation.
https://aka.ms/autorest
   Loading AutoRest core      '/root/.autorest/@microsoft.azure_autorest-core@2.0.4280/node_modules/@microsoft.azure/autorest-core/dist' (2.0.4280)
   Loading AutoRest extension '@microsoft.azure/autorest.nodejs' (~2.1.25->2.1.59)
   Loading AutoRest extension '@microsoft.azure/autorest.modeler' (2.3.44->2.3.44)

[AutorestCI]

Automation for azure-sdk-for-java

Encountered a Subprocess error: (azure-sdk-for-java)

Command: ['/usr/local/bin/autorest', '/tmp/tmpsm9c0zlo/rest/specification/web/resource-manager/readme.md', '--azure-libraries-for-java-folder=/tmp/tmpsm9c0zlo/sdk', '--java', '--multiapi', '--use=@microsoft.azure/autorest.java@2.1.71', '--verbose']
Finished with return code -9
and output:

AutoRest code generation utility [version: 2.0.4280; node: v8.11.3]
(C) 2018 Microsoft Corporation.
https://aka.ms/autorest
   Loading AutoRest core      '/root/.autorest/@microsoft.azure_autorest-core@2.0.4280/node_modules/@microsoft.azure/autorest-core/dist' (2.0.4280)
   Loading AutoRest extension '@microsoft.azure/autorest.java' (2.1.71->2.1.71)
   Loading AutoRest extension '@microsoft.azure/autorest.modeler' (2.3.38->2.3.38)
Processing batch task - {"tag":"package-2018-02"} .