run: escape arguments with spaces in it

```
#!/bin/bash

set -e
set -x

rm -rf myrepo
mkdir myrepo
cd myrepo

git init
dvc init

echo 'echo $1' > script.sh
chmod +x script.sh

./script.sh arg1 arg2
./script.sh 'arg1 arg2'

dvc run ./script.sh 'arg1 arg2'
```

`arg1 arg2` are passed as a single argument to dvc, but we join() it in dvc/command/run.py, without escaping it, causing such behavior.