SERVER-15500 SERVER-15515 system.users tests

This re-applies (with fixes) the following commits:
    0a98d654a7054d252dd921d27a5649f7177c8723.
    9e58312f0363b8164b59dc2541b733d7417a9986.

Closes #815

Signed-off-by: Benety Goh <benety@mongodb.com>
Merge remote-tracking branch 'upstream/v2.6' into v2.6