EUCA-8130, EUCA-8131: Protocol for distributing IAM server certificate/pk.
Background: To implement ELB SSL termination, we need a way to distribute user's private key (part of server certificate) to VMs.
    The proposed protocol is to use the existing trust chain between CLC and NC to deliver a random RSA keypair to the VM.
    The keypair is used for encryption/decryption of the server certificate exchanged between VMs and Euare.
    More detail of the protocol can be found: https://eucalyptus.atlassian.net/wiki/display/ARCH/ELB+SSL+Specification

The summary of changes is as follows:
  * Added SignCertificate method to EuareService. Note this is the euca-specific operation to sign a credential (public key),
    which is later used to access Euare-controlled resources (e.g., IAM server certificate).
  * setupCredentialMessages method is added to ClusterAllocator.java. For every runInstance request, the method examines if the instance's user-data
    begins with special keyword that indicates the credential-setup is required for the instance. If the keyword is found, it generates a random RSA
    pub/pk. The private key is encrypted using a symmetric key, which is encrypted using the NC's public key. The keys and the signature from Euare is
    concatenated and set to the 'credential' field of VmRunType.
  * setLoadBalancerListenerSSLCertificate is implemented on ELB
  * ELB triggers instance-credential transfer by putting the special keyword on user-data.
  * Cc/nc wsdl and the stubs are updated to add "credential" field to RunInstances request.
  * euca_auth.c is updated to support decryption using symmetric key algorithm.
  * Nc is updated to add floppy to Linux instances if credential setup is requested. The instance's pubkey, decrypted private key, signature (by Euare),
    and Euare's pubkey are added to floppy.