Add a wadl target to the documentation

Thanks to sphinxcontrib-docbookrestapi, the documentation for the REST API can
be automatically generated. Add a target to do this.

Change-Id: I9dda41bc00160bf1be1e275d9ac94a3a53571688
Remove redundant code in nova_client.Client

'a and a or b' is equal to 'a or b', the redundant code can be
removed.

This patch also fixes the docstring of __init__, which actually
initializes a nova client object, not returns.

Change-Id: I0815928a2b597452048a1cc6c2170259a53ccaf9
Closes-Bug: #1264099
Merge "Add a wadl target to the documentation"
Merge "Remove redundant code in nova_client.Client"