(#12458) default to users primary group, not root, in asuser

When only a user (no group) was given to the asuser method, the egid wouldn't
be changed and would remain the privileged gid. This has been changed to the
primary gid of the user.

Fix and analysis by Dominic Cleal <dcleal@redhat.com>

Contributed: Daniel Pittman <daniel@puppetlabs.com>
Contributed: Deepak Giridharagopal <deepak@puppetlabs.com>
Contributed: Nick Lewis <nick@puppetlabs.com>