restapi : Rename power management agent user_name in rsdl(#1005751)

updating hosts powermanagent fails from cli. The correct
option for agent.user_name is agent.username.

Fixes the parameter name in rsdl

Change-Id: Ie3bdab3a775e5280d1d16ef3409c9eb420ec43ef
Bug-Url: https://bugzilla.redhat.com/1005751
Signed-off-by: Ravi Nori <rnori@redhat.com>