Refactor FsAdaptor startPoint validation via method extraction.

In order to support the changes for the following bugs:

b/6513820: Enumerate share points or Namespaces given a hostname
b/12897821: Enumerate Namespace target links

Supplying only hostname or DFS Namespace as a startpoint
could eventually resolve to multiple shares being traversed.
In anticipation of these changes, this refactoring alters the
FsAdaptor.init code, extracting the startPoint validation into
a separate method.  This change also coalesces pulling the
configuration options out of the config.

Code Review: http://codereview.appspot.com/178320043