luci-app-statistics: fix APC UPS host configuration

Ensure that list of hosts is stored as space separated list, like it was
done in the previous Lua implementation.

Fixes: #5010
Fixes: e7d22dce

 ("luci-app-statistics: convert collectd configuration to client side views")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>