Change parameter name --elastic-pool to --dest-elastic-pool on az db copy

[edernucci]

Describe the bug
The parameter --elastic-pool refer to destination pool, not source.

To Reproduce
az sql db copy --dest-name feature-cep --name feature-cep --resource-group desenvolvimento --dest-resource-group production --dest-server feature-production --server feature-develop --elastic-pool feature-production

Expected behavior
az sql db copy --dest-name feature-cep --name feature-cep --resource-group desenvolvimento --dest-resource-group production --dest-server feature-production --server feature-develop --dest-elastic-pool feature-production

Environment summary
Linux greenpass 4.15.0-36-generic #39-Ubuntu SMP Mon Sep 24 16:19:09 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Additional context
Not applicable.

[jaredmoo]

Thank you for the suggestion! This was an intentional design decision. A database is uniquely identified by resource group name, server name, and database name. Therefore, these parameters are the ones that are needed for both source & dest. All other arguments specified for az sql db copy are pertaining to the destination.