Add rm context command

[jessicalins]

I thought would be nice to have a command that removes the context, without the need to individually remove tenant and/or api. This PR adds the following behavior:

• obsctl context rm <api>/<tenant> is the command
• It removes a tenant from an api. If the api has only one tenant, the whole api is removed
• It sees context as the relation between api+tenant. If an api does not have a tenant, the rm command cannot be used. The command obsctl context api rm <api> can be used though.

If we agree on this behavior, I'd then add tests + update docs :) It partially implements https://github.com/observatorium/obsctl/issues/23

Signed-off-by: Jéssica Lins jessicaalins@gmail.com

[douglascamata]

It removes a tenant from an api. If the api has only one tenant, the whole api is removed

I think there has to be some extra care when writing documentation and talking about this. With this sentence, some may understand that there is an API in Observatorium to manage tenants and that obsctl can interact with it to add/remove tenants... which is not the case.

[jessicalins]

@saswatamcode @douglascamata thanks for the review! I've added test + modified the docs. I think we can improve docs in another pr (related issue: https://github.com/observatorium/obsctl/issues/28)