Update user_id length to match Keystone schema in volume_usage_cache

Update the maximum length to match the Keystone schema for maximum
user_id length in volume_usage_cache. In the case that Keystone were
to leverage the full varchar 64, there would potentially be data loss
and/or collision of user_ids across records.

This change originates from the conversation about increasing the
length of user_ids in Keystone:

http://lists.openstack.org/pipermail/openstack-dev/2014-February/028125.html

Even with the length not changing, it is important to ensure there is no
loss of data/resolution when relying on user_id to identify users uniquely.

Closes-Bug: #1226171
Change-Id: I05a5644a29d6e2432311c2ee5331970d5e8b0683
Merge "Update user_id length to match Keystone schema in volume_usage_cache"