SERVER-15500 New test for system.user operations

Closes #809

Signed-off-by: Benety Goh <benety@mongodb.com>
SERVER-15515 user updates in mixed version replSet

Closes #810

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