can't parse image name with underline

[chuanchang]

Ⅰ. Issue Description

can't parse image name with underline and got error like this "Error: invalid reference"

Ⅱ. Describe what happened

as Description

Ⅲ. Describe what you expected to happen

can parse image name with underline

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

1. pouch pull registry.hub.docker.com/library/foo_
   or
2. pouch pull registry.hub.docker.com/library/foo__bar

Ⅴ. Anything else we need to know?

$ pouch pull registry.hub.docker.com/library/foo_
Error: invalid reference

$ pouch pull registry.hub.docker.com/library/foo__bar
Error: invalid reference

$ pouch pull registry.hub.docker.com/library/busybox:foo_
Error: invalid reference

$ pouch pull registry.hub.docker.com/library/busybox:foo__bar
Error: invalid reference

Ⅵ. Environment:

• pouch version (use pouch version):
  $ pouch version
  BuildTime: 2018-08-16T12:48:25+00:00
  GitCommit: 33dd176
  GoVersion: go1.9.1
  KernelVersion: 3.10.0-327.ali2012.alios7.x86_64
  Os: linux
  Version: 1.0.0-rc1
  APIVersion: 1.24
  Arch: amd64

• OS (e.g. from /etc/os-release):
  $ cat /etc/os-release
  NAME="Alibaba Group Enterprise Linux Server"
  VERSION="7.2 (Paladin)"
  ID="alios"
  ID_LIKE="fedora"
  VERSION_ID="7.2"
  PRETTY_NAME="Alibaba Group Enterprise Linux Server 7.2 (Paladin)"
  ANSI_COLOR="0;31"
  CPE_NAME="cpe:/o:alibaba:enterprise_linux:7.2:GA:server"
  HOME_URL="https://www.aliyun.com/"
  BUG_REPORT_URL="http://bbs.aliyun.com/"

ALIBABA_BUGZILLA_PRODUCT="Alibaba Group Enterprise Linux 7"
ALIBABA_BUGZILLA_PRODUCT_VERSION=7.2
ALIBABA_SUPPORT_PRODUCT="Alibaba Group Enterprise Linux"
ALIBABA_SUPPORT_PRODUCT_VERSION=7.2

• Kernel (e.g. uname -a):
  $ uname -a
  Linux e010125004158.bja 3.10.0-327.ali2012.alios7.x86_64 add travis.yml for CI #1 SMP Mon Oct 9 14:09:14 CST 2017 x86_64 x86_64 x86_64 GNU/Linux

• Install tools:

• Others:

[fuweid]

Actually, the _ is separator so that it can't be the ending of reference.

The link is here. https://github.com/opencontainers/image-spec/blob/fc936c78346d017dcdbc6dd220a8ff60675ce3b5/annotations.md#L31

@chuanchang