file.upload set policy explicitly

Summary:
This is pretty basic allowing a user to set the
policy as a valid string ('no-one' or 'users') or
as a valid PHID.  Without an explicit policy
a permissive one is set.

Test Plan:
Tested using the python-phabricator module (very basic api wrapper).

The arc cli syntax was evading me.

```import base64
from phabricator import Phabricator
phab = Phabricator()
with open('mypic.jpg') as f:
    encoded = base64.b64encode(f.read())

//set no-one as viewer which really means author only?
phab.file.upload(name='mypicnoone.jpg',
                 data_base64=encoded,
                 viewPolicy='no-one')

//set a specific phid as policy in this case a project
phab.file.upload(name='mypicphid.jpg',
                 data_base64=encoded,
                 viewPolicy='PHID-PROJ-fgvvnafmhvkgn2d5a4rf')

//no set policy ends up as 'users' i.e. ('all users')
phab.file.upload(name='mypicdefault.jpg', data_base64=encoded)```

Not able to really test canCDN attribute but it should be
fine and I tried to make it all consistent with D10166

Reviewers: 20after4, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: 20after4, epriestley, Korvin

Maniphest Tasks: T5685

Differential Revision: https://secure.phabricator.com/D10164