fix inventory.basedir is not absolute path when hostfile is startswith "./"

Fix a problem when 
ansible.cfg looks like 
```
[defaults]
hostfile = ./ansible_hosts
```
The inventory_dir is set to '.',  not the absolute path . 

That cause problem when I try to use lookup plugin like following:
some vars file 
```
somevar: "{{ lookup('file', inventory_dir + '/cred/server.crt') }}"
```
inventory_dir is resovled to '.',  the playbook will try to look for  `cred/server.crt` int the role dir , not in inventory_dir .