[Backend] Define swagger contract and create back implementation for update user

[brauliodiez]

We have already implemented POST and it contains a field that is not taking into consideration in the update (the password field), but the rest should be similar, it should receive the Id of the user to be update (in the create this is not taken into consideration).

Steps:

• Define swagger.
• Create nodejs implementation (validation here, if email does exists check that is the email of the current user).
• Create mock server implemetation.