Enhance linked services for MySQL, PostgreSQL, Teradata and DB2

[gadongji]

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[gadongji]

For MySQL and PostgreSQL, we are introducing a new property, connectionString, to replace all the previous properties.
For Teradata and DB2, the property, schema, is removed since it is already deprecated according to our docs.

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[sarangan12]

Please fix the errors reported in https://travis-ci.org/Azure/azure-rest-api-specs/jobs/355678382 for approval

[gadongji]

Hi @sarangan12, the errors are not caused by my changes I think. Or could you please point out how I need to fix it.
BTW, please hold on a while before merge this PR into master, thanks.

[lmazuel]

@AutorestCI rebuild azure-sdk-for-python

[sarangan12]

@gadongji The changes look ok to me. Do you want this merged or should I still hold?

[hvermis]

@sarangan12 please go ahead and merge.

[gadongji]

@sarangan12 @hvermis please still hold, we want to wait until UX ready. Will let you know when we want to merge :) Thanks.

[sarangan12]

STill waiting

[gadongji]

Yes, please still hold. Thanks.

[sarangan12]

@gadongji Any Updates

[gadongji]

@sarangan12 please still hold :)

[gadongji]

@sarangan12 please help complete this PR now. Our UX is ready now and thanks for your patient. :)

[gadongji]

Hi @sarangan12, could you please help merge this PR now? Thanks very much.

[gadongji]

Hi @sarangan12, please merge this PR at your first convenience :)

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