Change signature of stop method.

Passing a boolean is now deprecated in favour of a hash, this
change is introduced to keep fog forward compatible with api
changes and to make the fog api more consistent.
Add support for expunging node in cloudstack.

The method takes a hash, if expunge is set to true in that hash
the vm is expunged immediately. Expunge is false by default.
Merge pull request #2856 from lndbrg/add-support-for-vm-expunging

Add support for expunging node in cloudstack.