Single quote env var values.

Fix for #1568 

Output before fix
```
$ bin/packer build tester.json
null output will be in this color.

==> null: Waiting for SSH to become available...
==> null: Connected to SSH!
==> null: Provisioning with shell script: /var/folders/n2/ntbnkxtx3vl20v0zrhlbpxrh0000gn/T/packer-shell914657977
    null: bash: line 1: world: command not found
Build 'null' errored: Script exited with non-zero exit status: 127

==> Some builds didn't complete successfully and had errors:
--> null: Script exited with non-zero exit status: 127

==> Builds finished but no artifacts were created.
```

To
```
$ bin/packer build tester.json
null output will be in this color.

==> null: Waiting for SSH to become available...
==> null: Connected to SSH!
==> null: Provisioning with shell script: /var/folders/n2/ntbnkxtx3vl20v0zrhlbpxrh0000gn/T/packer-shell237057071
    null: hello world
Build 'null' finished.

==> Builds finished. The artifacts of successful builds are:
--> null: Did not export anything. This is the null builder
```