Close the watcher instance in the plugin after event is received.

[govint]

When running a stress test (500 volumes created concurrently between two docker hosts and within a single one) the plugin logs show below error in creating the watcher instance.

2017-03-24 06:40:04.419947253 +0000 UTC [INFO] Attaching volume and creating filesystem name=volTestP1349 fstype=ext4
2017-03-24 06:40:04.420212472 +0000 UTC [ERROR] Failed to create watcher. Skipping inotify Name=volTestP1349

lsof shows the plugin has a lot of open files via inotify
docker-vo 4856 root 6u unix 0x0000000000000000 0t0 985592 /run/docker/plugins/vsphere.sock type=STREAM
docker-vo 4856 root 7r a_inode 0,10 0 14 inotify
docker-vo 4856 root 8r a_inode 0,10 0 14 inotify
docker-vo 4856 root 9r a_inode 0,10 0 14 inotify
docker-vo 4856 root 10r a_inode 0,10 0 14 inotify
docker-vo 4856 root 11r a_inode 0,10 0 14 inotify
docker-vo 4856 root 12r a_inode 0,10 0 14 inotify
docker-vo 4856 root 13r a_inode 0,10 0 14 inotify
docker-vo 4856 root 14r a_inode 0,10 0 14 inotify
docker-vo 4856 root 15r a_inode 0,10 0 14 inotify
docker-vo 4856 root 16r a_inode 0,10 0 14 inotify
docker-vo 4856 root 17r a_inode 0,10 0 14 inotify
.....

watcher.Close() needs to be called to ensure cleanup is done per watcher instance.

[govint]

Closed via #1081