Parse the ${NAMESPACE} format string in a spec's format method. (#9686)

This allows installing software on a namespace basis by including ${NAMESPACE} in `install_path_scheme`. e.g.,

```
cat ~/.spack/config.yaml
config:
  install_path_scheme:
      "${ARCHITECTURE}/${NAMESPACE}/${COMPILERNAME}-${COMPILERVER}/${PACKAGE}-${VERSION}-${HASH}"
```

lib/spack/spack/spec.py

@@ -2944,6 +2944,7 @@ def format(self, format_string='$_$@$%@+$+$=', **kwargs):
             ${SPACK_INSTALL} The default spack install directory,
                              ${SPACK_PREFIX}/opt
             ${PREFIX}        The package prefix
+            ${NAMESPACE}     The package namespace
 
         Note these are case-insensitive: for example you can specify either
         ``${PACKAGE}`` or ``${package}``.
@@ -3126,6 +3127,8 @@ def write(s, c):
                     else:
                         hashlen = None
                     out.write(fmt % (self.dag_hash(hashlen)))
+                elif named_str == 'NAMESPACE':
+                    out.write(fmt % token_transform(self.namespace))
 
                 named = False