Fixes potentially deleting a null server id.
Remove Nova instance metadata limit

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
JCLOUDS-510 Delete objects in a container efficiently.

The existing approach for deleting objects in a container suffers
from a head-of-line blocking problem. This commit implements a better
scheme which does not have that problem. This scheme uses a counting
semaphore for making sure that a certain number of futures are
issued in parallel. As each of these futures is completed, one
permit of the semaphore is released.

Added unit tests for testing this new scheme.
JCLOUDS-516: Add ssh agent support via sch agentproxy
JCLOUDS-347: Implement a poor-mans rollback if CloudStack static NAT creation fails
Merge remote-tracking branch 'upstream/master'