Bump ansible in /src/config/fabric-ansible/ansible-playbooks

Bumps [ansible](https://github.com/ansible/community) from 2.6.13 to 2.6.18.
- [Release notes](https://github.com/ansible/community/releases)
- [Commits](https://github.com/ansible/community/commits)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

src/config/fabric-ansible/ansible-playbooks/requirements.txt

@@ -2,4 +2,4 @@ contrail-config-common
 contrail-api-client
 sandesh
 sandesh-common
-ansible==2.6.13
+ansible==2.6.18