Merge branch '2.x' into 2.x_n
fix netunmout do not refresh sometime
[cmd:netunmout] support VolumeDriver::netunmount() method
Merge branch 'netmount' into dropbox
[driver:dropbox] support netunmount() method

Drop table `dropbox` & rm thumbs on netunmount
Merge branch 'dropbox' into 2.x_n
Merge branch '2.x_n' into 2.1_n