Improves the metering doc. Agent/Plugin conf.

Add the metering agent to the list of network agents table.
Also add a section to specify the service plugin which needs
to be used to enable the metering on plugin side.

Not backported doc/admin-guide-cloud/ch_networking.xml

backport: havana
Partial-bug: #1202967
Change-Id: If12b655e3459cc0ae968fce4189db6623d2c3921
(cherry picked from commit 5a3bccd03b35fe19dba124d9688b890851c42cc1)
Merge "Improves the metering doc. Agent/Plugin conf." into stable/havana