Policy modifications should use PUT instead of POST

[dmoranj]

Currently, policy creation behaves as follows: "when a POST request with a policy arrives to the PAP:

• If the PolicyId already exists, it is updated with the contents of the request
• If the PolicyId does not exist, it creates a new policy"

This behavior is misleading, and should be changed, so that POST request with the same PolicyId end up in an error, and a PUT method should be added to deal with modifications.

[AlvaroVega]

The doc should be also updated: https://github.com/telefonicaid/fiware-keypass/blob/develop/API.md#create-or-update-a-policy