pimd, yang: Implement igmp static-group command

pimd, yang: Implement igmp static-group command

This will add a static IGMP group that does not rely on an underlying
socket join which sends traffic to the cpu unneccesarily. Instead, the
groups are joined directly without any IGMP interactions.
New command is under interfaces, 'ip igmp static-group ...'.
Added an alias for 'ip igmp join ...' to 'ip igmp join-group'.
Moved IGMP join groups to new yang list "join-group" and reused
the "static-group" list for the IGMP static groups.

Signed-off-by: Nathan Bahr <nbahr@atcorp.com>