Add method to parse XML booleans

Currently we don't convert boolean objects returned by the server to the
Ruby representation of booleans. This complicates further treatment of
these values, as they have to be treated as strings. To avoid that this
patch introduces a new "parse_bool" method in the base object class
intended to convert the boolean representation used by the server into
the Ruby representation. This method will be used in later changes.

Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>
Add support for Data Center "local" attribute

In version 3.4 of oVirt Engine the concept of type of a Data Center has
been relaxed, so that multiple types of storage (local, NFS, Fibre
Channel, etc) can be used in the same Data Center. So Data Centers no
longer have a type, but a "local" flag tha indicates if the storage is
only locally accessible or if it may be shared by multiple hosts. This
flag isn't currently available in the Ruby API. This patch adds it,
preserving the existing "storage_type" attribute for backwards
compatibility.

Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>