Expand variables in ssh-keypath from a node #780

expand embedded properties in SSH keypath and storage path, for node attributes, or project/framework defaults.

allows project/node attributes within a keypath like:

```{.xml}
<node name="mynode" environment="staging" ssh-keypath="/keys/${node.environment}.id_rdsa" />
```

or within a project.properties:

```
project.ssh-keypath=/keys/${node.environment}.id_rdsa
```