wskdeploy does not work with .rs extension rust files.

Rust 1.34 support works with https://github.com/apache/openwhisk-wskdeploy/pull/1119, however, this MR only passes tests for cargo projects in .zip format. Plain .rs files do not work as the runtime is not properly connected with the extension.

For the case of the Travis test pipeline, the problem is that the runtime tests are hardcoded to use the bluemix host: https://github.com/apache/openwhisk-wskdeploy/blob/master/runtimes/runtimes_test.go#L33

We should remove the hardcoded external dependency from the test and add test cases for .rs file.