Make 2018-01-01 as default for all clients

[solankisamir]

Make package-2018-01 as the default for all clients. package-2018-06-preview is still in development and can have breaking changes. Hence we dont want any clients to be built on top of that.

Inspired from the comment Azure/azure-sdk-for-net#4652 (review)

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Azure/azure-sdk-for-python#3106

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[solankisamir]

Please see: I did not change anything for the Java Client, still, they are appearing as diff. Just changed the Basic Information

@dsgouda FYI

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Can one of the admins verify this patch?

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Azure/azure-sdk-for-node#3310