the key of aggregate's metadata accepts empty string as key

[Lee Li]

Use command: nova aggregate-set-metadata llg_aggregate ''='test_key_empty'

it returns:

+----+---------------+-------------------+-------+--------------------------+
| Id | Name | Availability Zone | Hosts | Metadata |
+----+---------------+-------------------+-------+--------------------------+
| 1 | llg_aggregate | None | [] | {u'': u'test_key_empty'} |
+----+---------------+-------------------+-------+--------------------------+

The empty key value is not meaningful, so we should add a limit to this.

[Joe Gordon (jogo)]

Do we reject empty strings for other API calls as well? If not then we shouldn't reject it here. This is part of the bigger API parameter validation story.

[Lee Li (lilinguo)]

The server's metadata API reject empty string, such as:
nova meta 888238e4-660b-4b87-8f76-8b25d2d86135 set ""="test empty key"
ERROR: 无效的元数据: Metadata property key blank (HTTP 400) (Request-ID: req-a208e709-e0d6-40e2-ad6b-70ba06a4ffc1)

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/63694