Adds ! defined() guards around a2mod declarations

If you attempt to include both Zuul and Jenkins master modules in a
single manifest, you will get an error about A2mod[rewrite] resource
already being declared, like so:

Duplicate declaration: A2mod[rewrite] is already declared in file
/home/ubuntu/os-ext-testing/puppet/modules/os_ext_testing/manifests/master.pp
at line 30; cannot redeclare at
/root/config/modules/zuul/manifests/init.pp:236 on node undef

This isn't noticed by upstream, because no server runs both a Jenkins
master and a Zuul server. However, in smaller systems, where both
Jenkins and Zuul can be installed on the same host, the problem becomes
apparent.

This patch fixes that issue by adding guards around the a2mod
declarations in the Zuul init.pp and Jenkins master.pp manifests.

Change-Id: I2a298eb09ec8da33caed9349f465b2d163e2c299
Closes-bug: #1281676
Merge "Adds ! defined() guards around a2mod declarations"