Extended snort rule add to allow content field

Extended snort rule add to allow content field

- Exposed the 'content' field in the GRPC server AddRules method
- Allows the 'MALWARE-CNC User-Agent ASafaWeb Scan' signature
in the community rules to be copied to local rules
- Above ensures more deterministic alerts by snort each time
the signature is hit
- Added here to support the SDC configuration guide, which details
how to add this scan rule via GRPC client script

Change-Id: I6945c1e500075444134543bb9eb6003a03f1d5cc
Signed-off-by: Eddie Arrage <eddie.arrage@huawei.com>