Fix create, update or delete account metadata method

Swift provides a single API to Create, Update or Delete
account metadata.
With different headers or values those operation can be
achieved. For example:
- delete metadata by passing metadata without value
- update metadata by passing updated value with same key

Details- http://developer.openstack.org/api-ref/object-storage/?expanded=create-update-or-delete-account-metadata-detail

But current service client for account has 3 different
method for these operation. create_, delete_ and create_and_delete_

To make it consistent with other service client and to have
single service client method per API, this patch merge those
methods.

Partially implements blueprint consistent-service-method-names

Change-Id: I098e4ccc0d7b443d77ee055f39958e983acf7113
7 files changed