add ability to use warning messages in settings tab

[Ivan Berezovskiy]

I think we should add ability to show warning messages in settings tab.
For example user forgot to specify any optional parameter on settings tab. We should show warning message to him that may be user need to specify that parameter.
Now we can show only error message, that doesn't allow to save settings.

For example:

parameter1:
            value: ""
            label: "parameter"
            description: "..."
            weight: 30
            type: "text"
            regex:
              source: '^\w+$|.*'
              warning: "You may be need to specify parameter1"

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/134473

[OpenStack Infra (hudson-openstack)]

Change abandoned by Julia Aranovich (<email address hidden>) on branch: master
Review: https://review.openstack.org/134473
Reason: Design should be changed.