Allowing user to override endpoint hostname / is_secure / port in the configuration file

With this code changes, the user would be allowed to override DynamoDB endpoint / port / is_secure parameters without modifying the code. 

Which can be very useful in some scenario, for example, using DynamoDB Local for unit testing.

Here is the example.
If you start your DynamoDB local on your port 8000, and have following BOTO_CONFIG:
```
[DynamoDB]
is_secure = False
endpoint = localhost
port = 8000
```
You would be able to connect to your dynamodb local using following code:
```
import boto.dynamodb2
conn = boto.dynamodb2.connect_to_region(YOUR_REGION)
# The connection is actually connecting to localhost:8000.
# if you supply the BOTO_CONFIG, otherwise it would connect whatever
# YOUR_REGION is set.
```