appservice: make SKU optional when creating an appservice plan

[mayurid]

Closes #1381

[mention-bot]

@mayurid, thanks for your PR! By analyzing the history of the files in this pull request, we identified @yugangw-msft, @tjprescott and @derekbekoe to be potential reviewers.

[derekbekoe]

I think we should use the reflection we do on the methods for this.
There was a bug there in the method definition.
In the method linked below, we should have sku='B1' instead of sku.

And then, we can remove default='B1' from the params file.

https://github.com/Azure/azure-cli/blob/master/src/command_modules/azure-cli-appservice/azure/cli/command_modules/appservice/custom.py#L252

[yugangw-msft]

I author'd the defaults in the _param.py so all related literals will be in the same file for easier update in future. I am fine to leverage reflections, since if there are mismatch, the test will catch it anyway.

[mayurid]

@derekbekoe : Addressed the feedback. @yugangw-msft : Thanks for helping with it

[derekbekoe]

@mayurid I added 'Closes #1381' to the PR description so the issue closes when the PR is merged.

link: https://github.com/blog/1506-closing-issues-via-pull-requests

[tjprescott]

LGTM