Clarify behavior of add_enclave transaction

[masomel]

Following the #259 (comment) thread, we should discuss the behavior of the add_enclave transaction in Sawtooth and CCF.

The main outstanding question from this thread is whether adding a duplicate enclave should be treated as an error or as a no-op. As part of addressing this issue, we should also add more corner cases of add_enclave to our eservice tests.

[prakashngit]

@marcelamelara Could you please clarify what is a "duplicate enclave" ? (and a little bit more about the issue that you see), thanks!