gluster: Added gluster hooks support

Gluster hooks enable users to run scripts on pre or post level of
gluster command execution. Pre scripts are executed before gluster
command execution and post scripts are executed after gluster
command execution.

Below new verbs are added to perform to list, enable and disable hooks.

New verbs:
* glusterHookEnable
* glusterHookDisable
* glusterHooksList
    Output structure:
        [{'name': HOOK-NAME,
          'status': STATUS,
          'type': MIME_TYPE,
          'command': GLUSTERCOMMAND,
          'level': HOOK-LEVEL,
          'md5sum': MD5SUM}]

Change-Id: I3918aa035d90967f1297dc7fadcf14b6a9385c45
Signed-off-by: Timothy Asir <tjeyasin@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/9671
Reviewed-by: Aravinda VK <avishwan@redhat.com>
Reviewed-by: Adam Litke <agl@us.ibm.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>