VMware: Update default task_poll_interval value

The task_poll_interval option is used as the polling interval of vCenter/ESX
task status. This patch updates the default value to 0.5s in order to reduce
the wait time for task completion. Similar change has been done in nova and
this change is needed to keep the driver config values consistent.

DocImpact
   Updated default value of task_poll_interval to 0.5 seconds.

Change-Id: I66b80c629e255d61047592455ca42632f93051b2
Closes-bug: #1278028
Merge "VMware: Update default task_poll_interval value"