Fix --limit with commas

Kayobe allows specifying a --limit argument, which is passed through to
Ansible. In some cases we wish to add an intersection with a group. This
allows us to reuse playbooks for the seed, overcloud etc.

For example, the lvm.yml playbook specifies a host list of
seed-hypervisor:seed:overcloud. When executed as part of a kayobe
overcloud host configure command, Kayobe passes a limit of overcloud. If
the user specifies a --limit argument, this gets intersected with the
overcloud limit: host1:&overcloud.

The problem happens if the user specifies multiple parts to the host
pattern in their limit using a comma, e.g. host1,host2. This results in
host1,host2:&overcloud. Ansible ignores the colon, and treats this as
host1 or host2:&overcloud.

The solution is to use a comma to join the patterns if the user has used
a comma: host1,host2,&overcloud

Change-Id: Ibe42fa372c6fa0c539d2c2b0e238601286dc213d
Story: 2008255
Task: 41111