This patch fixes a discrepancy between NEO/py and NEO/go: NEO/py expands
the '~' and the '~username' prefix in the file path of the TLS
certificate/key files [1]. This syntax is used in NEO/py SlapOS SR [2].
We need to fix this discrepancy in NEO/go in order to use TLS encryption
with NEO + WCFS.

[1] https://lab.nexedi.com/nexedi/neoppod/blob/7c539f0f/neo/lib/config.py#L149
and https://lab.nexedi.com/nexedi/neoppod/blob/fa63d856/neo/lib/app.py#L25-31

[2] https://lab.nexedi.com/nexedi/slapos/blob/397726e1/stack/erp5/instance-zodb-base.cfg.in#L18-20
and https://lab.nexedi.com/nexedi/slapos/blob/a8150a1a/software/neoppod/instance-neo-input-schema.json#L62

/reviewed-by @kirr
/reviewed-on !1