feature: add image proxy info in 'pouch info'

[ZouRui89]

Signed-off-by: Zou Rui 21751189@zju.edu.cn

Ⅰ. Describe what this PR did

This pr enhance the output of 'pouch info' by adding the info of http proxy(image proxy).

Ⅱ. Does this pull request fix one issue?

fixes one part of #1096

Ⅲ. Describe how you did it

Ⅳ. Describe how to verify it

start pouch like
pouchd --image-proxy=http://127.0.0.1:5678

then

$ pouch info
Containers: 1
 Running: 1
 Paused: 0
 Stopped: 0
Images:  0
ID:
Name:
Server Version: 0.3-dev
Storage Driver:
Driver Status: []
Logging Driver:
Cgroup Driver:
runc: <nil>
containerd: <nil>
Security Options: []
Kernel Version: 3.10.0-693.17.1.el7.x86_64
Operating System:
OSType: linux
Architecture:
HTTP Proxy: http://127.0.0.1:5678
HTTPS Proxy:
Registry: https://index.docker.io/v1/
Experimental: false
Debug: true
Labels: []
CPUs: 0
Total Memory: 0
Pouch Root Dir: /var/lib/pouch
LiveRestoreEnabled: false
Daemon Listen Addresses: [unix:///var/run/pouchd.sock]

Ⅴ. Special notes for reviews

[allencloud]

We already has HttpProxy in SystemInfo struct.
Is it necessary for us to add an ImageProxy as well?
@Letty5411 @yyb196

[allencloud]

Please change this pull request ASAP, since it is quite urgent.

[ZouRui89]

Updated. PTAL @allencloud

[allencloud]

LGTM