Limits for IAM access keys and signing certificates - EUCA-9940

Cloud properties are added for limiting the allowed number of access key
and signing certificate credentials per user. The default value is two
for AWS compatibility:

 authentication.access_keys_limit           2
 authentication.signing_certificates_limit	2

An internal AuthorizationProvider interface is added to allow services
to dynamically provide Authorizations for evaluation along with those
from IAM policies. There is a PolicyAuthorizationProvider that provides
the Authorizations defined in policies. AccessKeyAuthorizationProvider
is added to provide an Authorization based on the cloud property for
access keys (and SigningCertificateAuthorizationProvider for signing
certificates)

EUCA-9940
EC2 ImportInstance action updates for VPC - EUCA-9934

Add support for subnet identifier and private IP address parameters with
ImportInstance.

EUCA-9934
Merge remote-tracking branch 'origin/dev/sjones/EUCA-9934' into testing
Merge remote-tracking branch 'origin/dev/sjones/EUCA-9940' into testing