provision-instance: remove dependency on PyYAML from pip

This isn't a built-in Python 3 module. It is an optional package
from pip, which this script doesn't locally install or provision.

```
$ bin/provision-instance.sh gruntjs-03.stage.ops.jquery.net staging
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'yaml'
```

Follows-up e037d5e.

bin/provision-instance.sh

@@ -6,7 +6,7 @@ set -euo pipefail
 INSTANCE=$1
 ENVIRONMENT=$2
 
-PUPPET_SERVER=$(python3 -c "import sys, yaml; print(yaml.safe_load(sys.stdin)['profile::puppet::agent::puppet_server'])" < hieradata/environments/"$ENVIRONMENT"/common.yaml)
+PUPPET_SERVER=$(cat hieradata/environments/"$ENVIRONMENT"/common.yaml | fgrep puppet_server | cut -d' ' -f2)
 VERSION_CODENAME=$(source <(ssh root@"$INSTANCE" cat /etc/os-release); echo "$VERSION_CODENAME")
 
 PUPPET="puppet"