RDKCOM-2599: RDKBDEV-711 IPV6: Fix incorrect preferred and valid lifetime
RDKCOM-2599: RDKBDEV-711 IPV6: Fix incorrect preferred and valid lifetime

Reason for change: Based on the code analysis, the PreferredLifetime and ValidLifetime
are hard-coded in theIPv6Prefix_Synchronize(). These values gets populated from
ip -6 addr show dev <interface> command output. The API _get_datetime_lfts() dumps this
command output to a file (/var/.ip_output)then _get_2_lfts() process this file and gets
PreferredLifetime and ValidLifetime.
Test Procedure: 1) Make sure the DUT is online.
2) Check the below datamodel via dmcli/webpa.
 Device.IP.Interface.1.IPv6Prefix.1.PreferredLifetime
 Device.IP.Interface.1.IPv6Prefix.1.ValidLifetime
3) Observe that The response should contain the time at which the lifetimes is
ending/until when the IPv6 prefix is valid.
Risks: None
Signed-off-by: Mohamed Shaikh <mohamed.shaikh@t-systems.com>

(cherry picked from commit 6ea5467b162b5cc7a86685660f0dad50bb4f1566)
Change-Id: Ic660f2f8e0fffe017759e42769b63829f1ac191b

rdkcmf/rdkb-CcspPandM,rdkcmf/rdkb-CcspPandM,rdkcmf/rdkb-CcspPandM