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.
Currently the only use of this `parse_bool` method is for the `local` attribute of Data Centers, and the server will never send anything other than `true` or `false` for that, so the exception will not happen. Even if the server send something incorrect the code is protected with `rescue nil`, so the user doesn't have to do anything. Throwing the exception is intended for other potential uses of the `parse_bool` method, where the user may actually want to detect incorrect values.