Use df(1) in a portable way

The _get_capacity_info() method of glance.store.filesystem.Store
class calls df(1) with '--block-size' GNU-speicifc argument
to get 1 byte blocks.

To make this code more portable [1], call df(1) with '-k' argument to
get 1024 byte blocks and multiply the result by 1024.

1: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/df.html

Change-Id: I7ab2eb1c8588fec3623046ddc91615ba2eba2c13
Merge "Use df(1) in a portable way"