JIRA HPCC 10637

Based on the second review the following modifications are made:

1) replaced /var/lib/juju/hpcc_data with /var/lib/HPCCSystems/charm
   All hpcc charm related user data saved in there. For example, ip files, url, etc

2) Add two string variables: ssh-key-public and ssh-key-private. User can generate
   ssh key pair and copy/paste the public key and private to these variables.
   install hook script will use the values if defined. Otherwise default ssh key pair
   will be used.

   It would be nice if we can generate key paris if user doesn't supply them. But we only
   want to have one pair of key fo reach clusters and not sure how to garrantte only
   one unit generating the key when relation changed. We will leave it to future enhancement.

3) README file is updated to reflect the change in 2)