integration tests: add env. variable for etcd URL.

Many integration tests delete all keys in etcd as part of their cleanup.
To run these tests in parallel we must run several etcd daemons, each on
different port and pass etcd url to the test suite.

test/integration/framework/master_utils.go

@@ -154,7 +154,7 @@ func startMasterOrDie(masterConfig *master.Config) (*master.Master, *httptest.Se
 // Returns a basic master config.
 func NewMasterConfig() *master.Config {
 	config := storagebackend.Config{
-		ServerList: []string{"http://127.0.0.1:4001"},
+		ServerList: []string{GetEtcdURLFromEnv()},
 		// This causes the integration tests to exercise the etcd
 		// prefix code, so please don't change without ensuring
 		// sufficient coverage in other ways.