cli: Add statusStr xml tag to task list and rebalance/remove brick status

New xml tag statusStr added to following gluster cli commands
gluster volume status all --xml (For Task status)
gluster volume rebalance <VOLNAME> status --xml
gluster volume remove-brick <VOLNAME> <BRICK1..> status --xml

Example(volume status all):
<task>
    <type>Rebalance</type>
    <id>82d8d122-8738-4144-8507-d93fc98b61df</id>
    <status>3</status>
    <statusStr>completed</statusStr>
</task>

Example(volume rebalance <VOL> status)
<node>
    <nodeName>localhost</nodeName>
    <files>0</files>
    <size>0</size>
    <lookups>0</lookups>
    <failures>0</failures>
    <status>3</status>
    <statusStr>completed</statusStr>
</node>

Also modified task status as string instead of showing number
in gluster volume status all

Example:
Status of volume: gv1
Gluster process                                         Port    Online  Pid
------------------------------------------------------------------------------
Brick sumne.sumne:/gfs/b1                               49154   Y       15489
Brick sumne.sumne:/gfs/b2                               49155   Y       15493
NFS Server on localhost                                 N/A     N       15913

           Task                                      ID         Status
           ----                                      --         ------
      Rebalance    82d8d122-8738-4144-8507-d93fc98b61df      completed

BUG: 1003521
Change-Id: Ib283016af4c18132fb13fb33d44075782d77823c
Signed-off-by: Aravinda VK <avishwan@redhat.com>
Reviewed-on: http://review.gluster.org/5739
Reviewed-by: Kaushal M <kaushal@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>