Implement describe_log_groups() method for CloudWatchLogs

This patch teaches `LogsResponse` class how to handle the DescribeLogGroups
request, so that we can mock out the `boto.describe_log_groups()` call.

With this change in place, we can write as below:

    @mock_logs
    def test_log_group():
        conn = boto3.client('logs', 'us-west-2')

        some_method_to_init_log_groups()

        resp = conn.describe_log_groups(logGroupNamePrefix='myapp')
        assert ...

This should be fairly useful for a number of programs which handles
CloudWatchLogs.

Signed-off-by: Fujimoto Seiji <fujimoto@clear-code.com>

tests/test_logs/test_logs.py

@@ -13,6 +13,10 @@ def test_log_group_create():
     conn = boto3.client('logs', 'us-west-2')
     log_group_name = 'dummy'
     response = conn.create_log_group(logGroupName=log_group_name)
+
+    response = conn.describe_log_groups(logGroupNamePrefix=log_group_name)
+    assert len(response['logGroups']) == 1
+
     response = conn.delete_log_group(logGroupName=log_group_name)