Use string literals when constructing config_vars

If there are any spaces in the envvars, the sourcing of `profile.d` scripts will fail with command not found errors.  For example, this fixes `.profile.d/config_vars: line 2: -Xss512k: command not found` when running a Java app.  

With the new quoting:
```
root@67ddd39d9783:/app# cat .profile.d/config_vars 
PATH='/app/.jdk/bin:/usr/local/bin:/usr/bin:/bin'
JAVA_OPTS='-Xmx384m -Xss512k -XX:+UseCompressedOops'
MAVEN_OPTS='-Xmx384m -Xss512k -XX:+UseCompressedOops'
```