Fix inventory_file variable in connection tests

[fullyint]

The default inventory = hosts (in ansible.cfg) is a directory.

Whereas in Ansible <= 1.9.4 the inventory_file variable returned hosts, in Ansible 2.0+ it returns null. The definition comes from self._inventory.src(), whose is_file() check apparently returned true in 1.9.4 but no longer does in 2.0+.

I initially thought we could just omit the -i inventory options from the affected commands and let the commands just use the default inventory specified in ansible.cfg. However, if a user specifies an actual file via the cli, the inventory_file variable will be non-null and our ssh connection tests should use that inventory. So, this PR adjusts commands to use the inventory_file variable only when it is not null.

The inventory_file needs the string filter so that when it is null the playbook will not fail with "ERROR! an unexpected type error occurred. Error was cannot concatenate 'str' and 'NoneType' objects".

[swalkinshaw]

👍