Useradd provider should support the supplementary groups (-G) option

Currently, one must first create the users and then add them to a group, which is fine for people who think about the problem in this order, but not for people who prefer to create the groups and then add users to them. Useradd(8) has a -G option for this purpose, but chef does not expose any way to use it.

http://linux.die.net/man/8/useradd

(original ticket description)

group "grp" do
       gid NNNN
        members ['user']
end
# fails if user does not exist

user "jboss" do
        uid NNN
        gid "grp"
end
# fails if grp does not exists

so group must be first and should not have not yet created users listed as members or it will fail.