Add function for parsing metadata in shell.py

[Zhenyu Zheng]

Currently, the metadata is parsed using dict(v.split('=', 1) for v in args.meta)
and it has been used in several places through the code.

It could be better to have a common function for this to make it more readable
and easy to use in the future.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/232301

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/232301
Committed: https://git.openstack.org/cgit/openstack/python-novaclient/commit/?id=09bb834b05106b36ff0ced47bfbd44ef915cbad6
Submitter: Jenkins
Branch: master

commit 09bb834b05106b36ff0ced47bfbd44ef915cbad6
Author: Kevin_Zheng <email address hidden>
Date: Thu Oct 8 09:38:34 2015 +0800

    Refactor parsing metadata to a common function

    Currently the metadata is parsed using
    dict(v.split('=', 1) for v in metadata)
    and was used in several places, it could
    be better if we refactor it to a common
    function. It will make it more readable
    and easier to use in the future.

    Change-Id: Ib0fcaacdbef65cdcb85ecc0c304f4933bef2627c
    Closes-Bug: #1503939