webadmin: fixed remove brick actions disabled issue in volumes tab

 When user move between sub tabs, entity of the previous tabs (all in
active tabs) being null. Because of this, menu items in volumes
tab are not getting enabled. So moved the volume entity as part of
VolumeBricksListModel and removed the usage of getEntity() in
all remove brick related commands.

Change-Id: I005592a1cda7d65cc069ad88f2c46d9f68b1f368
Bug-Url: https://bugzilla.redhat.com/1022585
Signed-off-by: Ramesh Nachimuthu <rnachimu@redhat.com>