Fix cpu_quota setting

After I30e550d21f00eb949ee2f3f8fb4d952c10bf8000, cpu_quota setting
(cpu_shares, cpu_quota, cpu_period) needs "quota" namespace.
If not specifying the namespace when customizing flavor, the quota
does not work.
This patch fixes the description.

Change-Id: I6dd8e921db665c62d5039283ec0ed2c433ccad2e