CLI image-update gives a wrong help on '--tags' param

The help message tells end user image-update interface accepts 'tags'
param and could be used to update tag of image [0], but actually it
could trigger an 400 exception [1] due to wrong PATCH calls, and the
correct way is to use image-tag-update interface [2] as designed.

[0] glance image-update <IMG_ID> --tags <TAG_VALUE>

[1] 400 Bad Request
    Invalid JSON pointer for this resource: '/tags/0'
        (HTTP 400)

[2] glance image-tag-update <IMG_ID> <TAG_VALUE>

Change-Id: Iaa8041779510192dc08f7b898b8a1beda29a6398
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "CLI image-update gives a wrong help on '--tags' param"