adding failover groups to swagger and changing failoverGroupId type

[btasdoven]

This change adds Failover Groups back to the public swagger list for Microsoft.Sql since I am in the process of adding .NET SDK tests for Failover Groups, and changes the type of failoverGroupId property in Database resource from Guid to string since this property represents the resource identifier of the failover group that contains the database.

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[btasdoven]

@jaredmoo, FYI, I sent this out as I am almost done with .NET SDK tests. I will follow up with a pull request for .NET SDK tests.

[AutorestCI]

Azure/azure-sdk-for-python#1136

[jaredmoo]

Already merged but looks great anyway :)

[AutorestCI]

Azure/azure-sdk-for-ruby#772

[AutorestCI]

No modification for NodeJS