pimd: Option to get IGMP groups and sources for a particular interface

pimd: Option to get IGMP groups and sources for a particular interface

1. Added interface name, group address and detail option to existing
   "show ip igmp groups" so that user can retrieve all the groups
   or a particular group for an interface. Detail option shows the source
   information for the group. With that, the show command
   looks like:

   "show ip igmp [vrf NAME$vrf_name] groups [INTERFACE$ifname [GROUP$grp_str]] [detail$detail] [json$json]"

2. Changed pim_cmd_lookup_vrf() to return empty JSON if VRF is not present

3. Changed "detail" option to print non pretty JSON

4. Added interface name and group address to existing
   "show ip igmp sources" so that user can retrieve all the sources for
   all the groups or, all the sorces for a particular group for an
   interface. With that, the show command looks like:

   "show ip igmp [vrf NAME$vrf_name] sourcess [INTERFACE$ifname [GROUP$grp_str]] [json$json]"

Signed-off-by: Pooja Jagadeesh Doijode <pdoijode@nvidia.com>