rgw/iam: add get_managed_policy() factory function

rgw/iam: add get_managed_policy() factory function

add definitions for the following managed policy ARNs:

* arn:aws:iam::aws:policy/IAMFullAccess
* arn:aws:iam::aws:policy/IAMReadOnlyAccess
* arn:aws:iam::aws:policy/AmazonSNSFullAccess
* arn:aws:iam::aws:policy/AmazonSNSReadOnlyAccess
* arn:aws:iam::aws:policy/AmazonS3FullAccess
* arn:aws:iam::aws:policy/AmazonS3ReadOnlyAccess

factory function get_managed_policy() returns a parsed Policy for the
requested ARN if available

Signed-off-by: Casey Bodley <cbodley@redhat.com>