Amazon splits credentials between DB Security and VPC Security Groups
for RDS instances. Fog currently does not parse the VPC security
groups. This patch adds that attribute to a Fog::AWS::RDS::Server.
Server#modify and #create_read_replica allow
options to be passed as symbols or strings.
This allows the same hash of attributes to be used
for creating & modifying servers.
For example, the following lines are equivalent:
server.modify(true, :allocated_storage => 10)
server.modify(true, 'AllocatedStorage' => 10)
