[bug] volume size doesn't work in diskquota test

[Letty5411]

Ⅰ. Issue Description

The following test failed:

11:24:10 FAIL: cli_run_test.go:911: PouchRunSuite.TestRunWithDiskQuotaRegular
11:24:10 
11:24:10 cli_run_test.go:968:
11:24:10     c.Assert(mount3Found, check.Equals, true)
11:24:10 ... obtained bool = false
11:24:10 ... expected bool = true

Ⅱ. Describe what happened

Steps to reproduce it:

#pouch volume create -n diskquota -o size=256m -o mount=/data/volume
Scope:
Status:       map[size:256m mount:/data/volume sifter:Default]
CreatedAt:    2018-5-24 11:46:48
Driver:       local
Labels:       map[]
Mountpoint:   /data/volume/diskquota
Name:         diskquota

#pouch run --disk-quota=1024m --disk-quota=".*=512m" --disk-quota="/mnt/mount1=768m" -v /data/mount1:/mnt/mount1 -v /data/mount2:/mnt/mount2 -v diskquota:/mnt/mount3  registry.hub.docker.com/library/busybox:1.26 df
Filesystem           1K-blocks      Used Available Use% Mounted on
overlay                1048576        44   1048532   0% /
tmpfs                    65536         0     65536   0% /dev
shm                      65536         0     65536   0% /dev/shm
tmpfs                    65536         0     65536   0% /run
tmpfs                131776220         0 131776220   0% /sys/fs/cgroup
/dev/sda3               786432         4    786428   0% /mnt/mount1
/dev/sda3               524288         4    524284   0% /mnt/mount2
/dev/sda3               524288         4    524284   0% /mnt/mount3
tmpfs                    65536         0     65536   0% /proc/kcore
tmpfs                    65536         0     65536   0% /proc/timer_list
tmpfs                    65536         0     65536   0% /proc/timer_stats
tmpfs                    65536         0     65536   0% /proc/sched_debug
tmpfs                131776220         0 131776220   0% /sys/firmware
tmpfs                131776220         0 131776220   0% /proc/scsi

#pouch volume inspect diskquota
[
    {
        "CreatedAt": "2018-5-24 11:46:48",
        "Driver": "local",
        "Labels": {
            "backend": "local",
            "hostname": "r10a07137.sqa.zmf"
        },
        "Mountpoint": "/data/volume/diskquota",
        "Name": "diskquota",
        "Status": {
            "mount": "/data/volume",
            "ref": "a8a2d3169df2a81ee4054ecfb1d9d9734704202a1d53fa74db3c8750c6286922,d3c095cdcdf6db823c63477eec61d0bcb608bfd28d922faf4ebf2307c4eb94be,cc72bb36755122992ee99227d19535981bfb15a717c352386724693ec5bb6f7c",
            "sifter": "Default",
            "size": "256m"
        }
    }
]

Ⅲ. Describe what you expected to happen

Ⅳ. How to reproduce it (as minimally and precisely as possible)

Ⅴ. Anything else we need to know?

Ⅵ. Environment:

• pouch version (use pouch version):
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others: