1.7.x Backport:  Remove Nova instance metadata limit

master:  https://github.com/jclouds/jclouds/pull/324

Rackspace cloud limits instance metadata to 5 key-value pairs, but
upstream Nova only sets the limit at 128 by default.  This patch removes
the limit entirely; the official python clients don't check it and the
server is responsible for enforcing it anyway.

Fixes:  https://issues.apache.org/jira/browse/JCLOUDS-507