test-example: Make sites.conf variable, in principle

test-example: Make sites.conf variable, in principle

In this commit we split the setting of the sites.conf into two
branches depending on the value of privkey($site).  The path prefix
is common - only the leafname varies.

Right now there isn't actually any change because both branches use
the same sites.conf.  The new-style arrangement needs a new-style
sites file which isn't there yet.

So no functional change.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>