Feature/cleanup api

Depends on: https://github.com/leapcode/leap_mail/pull/152
Interrelated with: https://github.com/leapcode/bitmask_client/pull/756 & https://github.com/leapcode/leap_pycommon/pull/92

_I'm not sure if commits should be rebased together_
> bitmask_client is not using it, see the 'return' in front. I was adding a comment to it with my commit there. I didn't know about soledad, if soledad uses it I guess I'll have to revert the removal.
> 
You're right, I didn't see that. If we restore that feature we will need this functionality though.
We should ask drebs about if this is needed in soledad.

> I don't like the functionality and name of this method, the rest of the class works as a key storage but this one does key parsing, that has nothing to do with the storage, it actually don't load the key in the storage. I'll give it more thought.

Indeed. If we still need this we should move it to a helper module instead of here.
Speaking for the client, if sometime we need this again we could have it on the utils module.