make amphorae's haproxy.cfg configurable

All amphorae are currently using the same haproxy configuration, which is rendered from the jinja templates installed by the octavia package. Some haproxy option are configurable via the Octavia API (e.g. max connections and several timeouts), but others are notably not (i.e. nbthread). This severely limits the possibility of performing performance tuning of octavia installations.

Octavia does however offer the possibility of passing a custom haproxy.cfg jinja template, so it would be very useful if the octavia charm exposed it, as that would allow operators to fully tune the haproxy configuration of their amphorae.