Change the default prefix

[tacaswell]

Currently the default prefix in the IOC is '' which is what we want to use in production.

The down side of this is that for testing we need to remember to pass --prefix=TEST to ensure we do not collide with the production PVs.

We should

1. change change the default prefix to be anything else (see below)
2. update the ansible role to explicitly set the prefix to be an empty string

archiver-proxy/archiver_proxy.py

Line 88 in 09d0b3c

default_prefix="",