Implementation of log group discovery on EC2 not work on Beanstalk

Hi all,

The feature [Implementation of log group discovery on EC2](https://github.com/aws/aws-xray-sdk-java/commit/08629ee2a61ea73d85c6af0843150fd7d9399617) released on the **AWS X-RAY SDK 2.4.0** and not described on the [CHANGELOG](https://github.com/aws/aws-xray-sdk-java/blob/master/CHANGELOG.md) does **not work** properly on Beanstalk.

```java
WARN com.amazonaws.xray.plugins.EC2Plugin.populateLogReferences(EC2Plugin.java:121) [] -
CloudWatch Agent log configuration file not found at /opt/aws/amazon-cloudwatch-agent/etc/log-config.json.
Install the CloudWatch Agent on this instance to record log references in X-Ray.

```

The code is searching for the "CloudWatch Logs" configuration on the following path HARDCODED.

https://github.com/aws/aws-xray-sdk-java/blob/c183a3aad20a528a69cff26d77dfb691f503edb9/aws-xray-recorder-sdk-core/src/main/java/com/amazonaws/xray/plugins/EC2Plugin.java#L44

The problem is that the **ElasticBeanstalk** platform uses the does not use the `/opt` as the CloudWatch Logs path, neither for it's configurations:

```
[root@ip-172-31-0-213 ec2-user]# ps aux | grep log
root 2528 0.8 1.2 913352 50876 ? SNl 09:22 0:13 /usr/bin/python2.7 /usr/bin/aws logs push --config-file /etc/awslogs/awslogs.conf --additional-configs-dir /etc/awslogs/config

[root@ip-172-31-0-213 ec2-user]# tree /opt/aws -L 1
/opt/aws
├── amitools
├── apitools
├── bin
└── credential-file-path.template
```
The above issue also caused the a miss on the feature #110 referred to the code below returning this message:

```java
WARN com.amazonaws.xray.AWSXRayRecorderBuilder.lambda$build$1(AWSXRayRecorderBuilder.java:249) [] -
com.amazonaws.xray.plugins.EC2Plugin plugin returned empty Log References.
The recorder will not reflect the logs from this plugin.
WARN com.amazonaws.xray.AWSXRayRecorderBuilder.lambda$build$1(AWSXRayRecorderBuilder.java:249) [] -
com.amazonaws.xray.plugins.ElasticBeanstalkPlugin plugin returned empty Log References.
The recorder will not reflect the logs from this plugin.
```

https://github.com/aws/aws-xray-sdk-java/blob/c183a3aad20a528a69cff26d77dfb691f503edb9/aws-xray-recorder-sdk-core/src/main/java/com/amazonaws/xray/AWSXRayRecorderBuilder.java#L243-L254

# Proposed Solution:
* Add a configuration to change the **LINUX_PATH** and **WINDOWS_PATH** for the custom directories
* Implement the **AWSLogReference** to cover EC2Plugin, and ElasticBeanstalkPlugin as it looks like this was designed to be a Lambda feature!