Handle 'groups=' (empty string) in user module (Linux)

Makes it possible to remove a user from all but the primary group.

Setting 'groups=' is the equivalent to passing '-G ""' to
useradd/usermod, which is interpreted as "no group" on Linux.
added thirsty option to copy
Signed-off-by: Brian Coca <briancoca+dev@gmail.com>
Merge branch 'devel' of git://github.com/trendels/ansible into testing
Merge branch 'thirsty_copy' of git://github.com/bcoca/ansible into testing
Slight docs tweak about thirsty mode on the copy module