Expect unlocking user w/o password to fail on sles

On all other linux distros, `usermod`, when attempting to unlock a user
with no password set, refuses to unlock the user, but exits with status
0. On SLES 11 and 12, `usermod` exits with status 3, which then causes
mixlib shellout to raise an exception. Make the tests differentiate
expected behavior by platform.
Expect user home dir to always be created on RHEL

- Inconsistent with other platforms because of etc/login.defs
- See CHEF-2205 http://tickets.opscode.com/browse/CHEF-2205
Expect user home dir to always be created on RHEL on more tests.
Merge pull request #847 from opscode/el-usermod-test-fixes

El usermod test fixes