Impacted versions: 11.0

Steps to reproduce: On runbot, 
- install pad_project module
- Change access rights on demo user to make him "project user"
- Connect on the same DB with demo user
- Create a task using form view with a project
![image](https://user-images.githubusercontent.com/7678583/43644317-a0bf9208-972e-11e8-9bb1-535c972083c8.png)
- Save


Current behavior:
Access right issue
![image](https://user-images.githubusercontent.com/7678583/43644338-af6fa5d6-972e-11e8-9d98-c24591587bcc.png)

The errors comes from related field use_pads:
![image](https://user-images.githubusercontent.com/7678583/43644403-dc73696e-972e-11e8-8175-75712b763a68.png)
By doing this the project user tries to update the project but the project is only accessible in read access

Expected behavior:
The task should be created

Video/Screenshot link (optional):


from odoo import api, fields, models
class ProjectTask(models.Model):
    _name = "project.task"
    _inherit = ["project.task", 'pad.common']
    description_pad = fields.Char('Pad URL', pad_content_field='description', copy=False)
    use_pad = fields.Boolean(related="project_id.use_pads", string="Use collaborative pad")
    @api.model
    def create(self, vals):
        
        project_id = vals.get('project_id', False) or self.default_get(['project_id'])['project_id']
        if not self.env['project.project'].browse(project_id).use_pads:
            self = self.with_context(pad_no_create=True)
        return super(ProjectTask, self).create(vals)
class ProjectProject(models.Model):
    _inherit = "project.project"
    use_pads = fields.Boolean("Use collaborative pads", help="Use collaborative pad for the tasks on this project.")