Update github.com/hashicorp/go-discover to pull in new config parser

This patch updates the go-discover library to use the new config parser
which uses a different encoding scheme for the go-discover config DSL.
Values are no longer URL encoded but taken literally unless they contain
spaces, backslashes or double quotes. To support keys or values with
these special characters the string needs to be double quoted and usual
escaping rules apply.

Fixes #3417

vendor/vendor.json

@@ -25,7 +25,7 @@
 		{"path":"github.com/hashicorp/errwrap","checksumSHA1":"cdOCt0Yb+hdErz8NAQqayxPmRsY=","revision":"7554cd9344cec97297fa6649b055a8c98c2a1e55","revisionTime":"2014-10-28T05:47:10Z"},
 		{"path":"github.com/hashicorp/go-checkpoint","checksumSHA1":"nd3S1qkFv7zZxA9be0bw4nT0pe0=","revision":"e4b2dc34c0f698ee04750bf2035d8b9384233e1b","revisionTime":"2015-10-22T18:15:14Z"},
 		{"path":"github.com/hashicorp/go-cleanhttp","checksumSHA1":"b8F628srIitj5p7Y130xc9k0QWs=","revision":"3573b8b52aa7b37b9358d966a898feb387f62437","revisionTime":"2017-02-11T01:34:15Z"},
-		{"path":"github.com/hashicorp/go-discover","checksumSHA1":"uNoQWG5h2hzWHjaLi376ZXVaCr4=","revision":"21b26b722865b64ae5809a532d2c18f3a3800129","revisionTime":"2017-08-16T16:53:52Z"},
+		{"path":"github.com/hashicorp/go-discover","checksumSHA1":"tWQmb13hmUBoWi0ZHnc4CLBqRc0=","revision":"745a463b035b25a4d0c12d056e208e3850fdd1be","revisionTime":"2017-10-04T16:46:45Z"},
 		{"path":"github.com/hashicorp/go-discover/provider/aws","checksumSHA1":"lyPRg8aZKgGiNkMILk/VKwOqMy4=","revision":"25e4565347de14cea0a0e0730374c9fcffa7bab0","revisionTime":"2017-09-25T01:06:15Z","tree":true},
 		{"path":"github.com/hashicorp/go-discover/provider/azure","checksumSHA1":"r97P32e+VmNMh2vwLkZa1zPEDQU=","revision":"b518491d039b6782035b8881502b4f5e9fcc887b","revisionTime":"2017-08-01T15:32:04Z","tree":true},
 		{"path":"github.com/hashicorp/go-discover/provider/gce","checksumSHA1":"KC/MepQsQF17904UShiM61jmaEs=","revision":"b518491d039b6782035b8881502b4f5e9fcc887b","revisionTime":"2017-08-01T15:32:04Z","tree":true},