Allow to customize the S3 direct upload token expiration

We would like to simplify how to tweak the S3 token TTL.

Currently, it can be done in 2 steps:

1. Tweak the value in nuxeo.conf:

# Value in seconds, here 10 hours
nuxeo.s3storage.transient.expiration=36000

2. Override the s3directupload-config.xml file from the nuxeo-amazon-s3 addon to add this line:

<property name="expiration">${nuxeo.s3storage.transient.expiration}</property>

The second step is too much, we should patch the template to include the required line.