Fix DB API mismatch with sqlalchemy API

We added a sqlalchemy columns_to_join argument to the
security_group_get_by_name() method, but didn't punch it through
the toplevel DB API. This patch adds that so that they match.

Change-Id: I6fe8d876834036e6fbd79d60f3d7d696d7cf4b83
Make SecurityGroup receive context

This fixes the SecurityGroup._from_db_object() method to take and set
a context, like most everything else.

Change-Id: Ie86df2cf264a1e75353599a8f9f693ef5fb15bc5
Merge "Fix DB API mismatch with sqlalchemy API"
Merge "Make SecurityGroup receive context"
xenapi: post install scriptlet to rpm

A freshly installed XenServer requires some modifications in order to be
able to run OpenStack. Nova contains an rpm specification to create a
package with the dom0 plugins, however installing the rpm is not enough.
Some more actions need to be taken:
 - create /images, link to a directory within the local SR
 - setup an ssh keypair, and authenticate it with the host
This change adds a post script to the nova plugins rpm spec file, so
by installing the rpm on a XenServer, it is ready to run OpenStack, no
further manual actions needed.

Change-Id: Id57e3a3ab2782550468c6aa4c01059ceb7cc81ab