webadmin: UI-plugins MainTabActionButton CommandLocation

Add ability to set button location using addMainTabActionButton API.

For example:

    addMainTabActionButton('Host', 'My Button', {
        location: 'OnlyFromContext'
    });

Supported values for 'location':
* OnlyFromContext
* OnlyFromToolBar
* ContextAndToolBar (default)

Change-Id: I034be5af1b2dcbaad4bfce9784d74410a92fb4be
Signed-off-by: Daniel Erez <derez@redhat.com>
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>