Added ability to poll an instance's metadata

* Added ability to poll a given server's metadata for a desired key/value
* Added TimeoutException message that relays the final value of key at timeout
* [removed] Added UndesiredValueException to allow users to watch for undesirable key:value pairs during polling

Change-Id: Iaf33b1bf0679fc7b5d27294e2a7a8fbe53bf8908
Merge "Added ability to poll an instance's metadata"