luci-app-statistics: treat APC UPS "host" setting as single value option

Existing Lua code incorrectly stated that the "Host" option takes a space
sparated list of hostnames which is not the case since the collect plugin
does not handle multiple hosts.

This change reverts the configuration to a simple value as proposed by
the original PR and adjusts the config summary accordingly, while retaining
the translation string.

Ref: https://github.com/openwrt/luci/pull/5010#issuecomment-827285319
Fixes: dd5d96af ("luci-app-statistics: fix APC UPS host configuration")
Fixes: e7d22dce

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