playbook: Support "offline" ansible runs

By setting default slapos_ansible_offline to false and using it as
conditional for repository role it is possible to drive, from example
from host variable, if playbook shall restrain itself from running
oneline required roles.

/reviewed-on !33

playbook/roles/slapos-proxy/defaults/main.yml

+---
+slapos_ansible_offline: False
\ No newline at end of file

playbook/roles/slapos-proxy/meta/main.yml

 ---
 dependencies:
-  - { role: repository } 
+  - { role: repository, when: not slapos_ansible_offline}