Add support for the Data Center "local" attribute

In version 3.4 of _oVirt Engine_ the concept of _storage type_ of a Data Center has changed. A Data Center used to allow only one type of storage (NFS, Fibre Channel, local, etc), but now different types of storage can be merged. However, a particular Data Center only allows _local_ storage (like locally attached SATA
disks, for example) which can't be shared by multiple hosts or _shared_ storage which can be shared by multiple hosts (like NFS, iSCSI or Fibre Channel). This is represented by a new `local` attribute, and the old  `storage_type` attribute has been deprecated.

This set of patchs adds support for this new `local` attribute, preserving the old `storage_type` attribute for backwards compatibility.

As the new `local` attribute is a boolean the set of patches also adds a new `parse_bool` method to the `BaseObject` class, intended to convert the strings returned by the server into the Ruby native boolean representation.
What in this case rbovirt's user is expected to do (When an argument error is thrown) ? It's a case when the server itself supplies incorrect information.
is this checked for other fields too?