Remove dependencies for netaddr.

Remove dependencies for netaddr.

Updates cinder utils.py to remove the parse_server_string and
is_valid_cidr functions. These functions aren't in use
and are the only reason we need the netaddr dependency.

Updates pip-requires to drop the netaddr dependency.

Also removes the unit tests for removed functions.
Change-Id: I521ae60b758324069fb1ffa1701b1f9deb780f65