Initialize shares variables for RemoteFsDriver(s)

self.shares and self._mounted_shares are used by various methods
for RemoteFs-based drivers, but may not be instantiated early enough.

Initialize them in __init__ to ensure they always exist.  (Also fixes
pylint warnings.)

Closes-Bug: #1239741
Change-Id: I0ef884c28b82b7f694cf8a0fb4629c835da11c91
Merge "Initialize shares variables for RemoteFsDriver(s)"