Bump ansible from 2.6.13 to 2.6.18 in /src/config/fabric-ansible

Bumps [ansible](https://github.com/ansible/ansible) from 2.6.13 to 2.6.18.
- [Release notes](https://github.com/ansible/ansible/releases)
- [Commits](ansible/ansible@v2.6.13...v2.6.18)

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

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

@@ -11,4 +11,4 @@ python-novaclient==16.0.0
 webtest==2.0.34
 contrail-api-server==0.1.dev0
 python-swiftclient==3.9.0
-ansible==2.6.13
+ansible==2.6.18