Add signing-service as an argument to repository creation and updation

[balasankarc]

Closes to #54

Implements --signing-service argument to pulp deb repository create command.

[balasankarc]

@quba42 @bmbouter I am not exactly sure how to handle signing_service_release_overrides that was added in pulp/pulp_deb#689, which takes in a dictionary as input. Do you have any suggestions, or does the project have any precedents?

[balasankarc]

@quba42 @bmbouter I am not exactly sure how to handle signing_service_release_overrides that was added in pulp/pulp_deb#689, which takes in a dictionary as input. Do you have any suggestions, or does the project have any precedents?

4593c2c is my way of implementing it. WDYT?

[balasankarc]

@quba42 Can you take a look? Thanks.

[mdellweg]

It might help to add some testing to see that it works.

[balasankarc]

It might help to add some testing to see that it works.

@mdellweg Agreed. I added some tests, inspired from pulp-cli.