Use docstrings for attributes in api/controllers

We're using one double-quote for delimiting the strings that describe
attributes. If the string spans more than one line, only the first line
is used when sphinx generates the documentation. eg, for last_error, the
generated documentation shows:
  Any error from the most recent (last) asynchronous transaction that

instead of
  Any error from the most recent (last) asynchronous transaction that
  started but failed to finish.

Using the docstring (three double-quotes) syntax addresses this. All the
attributes in api/controllers were modified accordingly. This makes them
consistent so that any additions/changes to these strings in the future
will appear in the generated documentation as expected. (As opposed to
only changing the ones that span more than one line.)

[1] http://docs.openstack.org/developer/ironic/webapi/v1.html

Change-Id: I84e4ba9c22827838ff31ba96a788cfc22609c8a5
Merge "Use docstrings for attributes in api/controllers"