make vagrant up work on fresh cloned repository

Hi, 

while evaluating ceph today I noticed that vagrant up is not working currently on a fresh clone of ceph-ansible.
Since I was realy interested in ceph and ansible I invested some minutes to fix it.

First change is a minor configuration change: monitor_interface: eth1 is  needed in group_vars/all.
The ssecond change is to use a more elegant way to wait for /etc/ceph/ceph.client.admin.keyring in roles/ceph-mon/tasks/main.yml, as using until as it was done before did not work in my case.

I hope I did not brake anything :) and that it helps other fresh ceph users and evaluators.

Cornelius