config: change valid tag to use regexp

A regular expression is much more useful than a list of strings.
config: add validity check for file permissions
validate: fix printing for non-string values
config: fix parsing of file permissions

These reintroduces the braindead '744' syntax for file permissions. Even
though this number isn't octal, it is assumed by convention to be. In
order to pull this off, coerceNodes() was introduced to try to
counteract the type inferrencing that occurs during the yaml
unmarshalling. The config is unmarshalled twice: once into an empty
interface and once into the CloudConfig structure. The two resulting
node structures are combined together. The nodes from the CloudConfig
process replace those from the interface{} when the types of the two
nodes are compatible. For example, with the input `0744`, yaml
interprets that as the integer 484 giving us the nodes '0744'(string)
and 484(int). Because the types string and int are compatible, we opt to
take the string node instead of the integer.