Cluster inventory refresh

[brainfunked]

The inventory structure has been defined in #8.

gadmin would determine the list of available clusters by checking for the following:

• Get the list of directories under $GADMIN_HOME/ansible/inventory/.
• For each of the directories, check if the file hosts.yaml exists.

The above steps give us an inventory 'refresh' step. The refresh would need to be carried out each time a new cluster is defined or an existing cluster is removed.

[sabose]

Same concern as #8 - we don't want to maintain a copy of state with gadmin