Added support for id in is_ssh_key data source

[uibm]

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Relates OR Closes #0000

Output from acceptance testing:

% make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISSSHKeyDatasource_basicidname'
=== RUN   TestAccIBMISSSHKeyDatasource_basicidname
--- PASS: TestAccIBMISSSHKeyDatasource_basicidname (47.26s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     49.344s