fix clients cannot connect to activemq middleware / broker network

The settings plugin.activemq.pool.${name}.ssl.cert and -.key were only
set in server::config::connector::activemq::hosts_iteration (called for
each broker in middleware_hosts on mcollective server). Servers could
connect to the broker network, clients not.
Patch moves the setting of plugin.activemq.pool.${name}.ssl.(cert|key)
to common::config::connector::activemq::hosts_iteration . Values stay
unchanged.