Fix ssh_keys template 'group cannot be determined' error

As discussed on https://github.com/opscode-cookbooks/users/issues/64 , I had some trouble using the 'ssh_keys' attribute when using the users cookbook...

When my user.json did NOT include a 'gid' attribute, I was getting an error:
```
template[/home/giorgos/.ssh/authorized_keys] (/tmp/vagrant-chef-3/chef-solo-1/cookbooks/users/providers/manage.rb line 121) had an error: Chef::Exceptions::GroupIDNotFound: cannot determine group id for 'giorgos', does the group exist on this system?
```
(https://github.com/opscode-cookbooks/users/blob/master/providers/manage.rb#L121)

The template resource was not able to find the group id...
So, I tried including a 'gid' attribute, only this time:
```
==> default: Chef::Exceptions::User
==> default: ----------------------
==> default: user[giorgos] (/tmp/vagrant-chef-3/chef-solo-1/cookbooks/users/providers/manage.rb line 94) had an error: Chef::Exceptions::User: Couldn't lookup integer GID for group name 7000
```
...leaving me no choice but to change the source code...
(This is all on an OpenSUSE 13.1 VM, so it might work on other OS)