Do not assign supplementary groups

Do not assign supplementary groups

Due to splitting password and group database to read-only db (standard
/etc/) and read-writable (/opt/etc/) it might no longer be possible to
add newly created users to system groups.  Precisely, adding user to
system given group requires appending user name to /etc/group, which
is on read only partition.  It's not possible to add such entry to
/opt/etc/group because it would not only require group name (and id)
duplication, but also it would require changing all the supporting
code to look for multiple group definitions.

To handle above problem gumd will no longer assign group membership at
user creation.  It will also use standard /etc/group (and gshadow).

Assiging users to correct groups will be handled by security-manager
nss plugin.

Change-Id: I86af2d41f07f13f0d0e6904cfb7b45fe84594ea7