Allow users to assign a security group to an app

https://review.openstack.org/455773
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/murano" is set up so that we directly report the documentation bugs against it. If this needs changing, the docimpact-group option needs to be added for the project. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit 103f67815a1cb2c514241fbf2e897684b5a269ae
Author: Paul Bourke <email address hidden>
Date: Tue Apr 11 17:26:42 2017 +0100

    Allow users to assign a security group to an app

    This patch allows users to supply a list* of their own security groups to
    an instance, rather than using the application defined one (built via
    the SecurityGroupManager).

    * Note, while we can support multiple security groups, murano-dashboard
    currently has no UI element to select multiple items. This means that
    currently users are restricted to selecting one group. If/when the
    UI is improved this change can easily support multiple groups.

    Example
    =======

    Application authors can make this available in their apps as follows:

    UI.yaml
    -------
    Forms:
      - instanceConfiguration:
          fields:
              ...
            - name: securityGroups
              type: securitygroup
              label: Security Group
              required: false

    Class.yaml:
    ----------
      Application:
        ?:
           type: com.paul.HelloWorld
        instance:
          ?:
            type: io.murano.resources.LinuxMuranoInstance
          name: $.instanceConfiguration.hostname
          securityGroups: $.instanceConfiguration.securityGroups
          ...

    DocImpact
    Change-Id: I60d37cfe034c467e894ee93cf3718e463bf49337
    Partially-Implements: blueprint app-use-existing-security-group