Login with 'centos' user for node launch

HP Cloud centos7 images are currently failing with 'Please login as
the user "centos" rather than the user "root"'.

This adds the 'centos' user as one of the logins to try, and adds a
little documentation to the subsequent fixup of the authorized_keys
file

Change-Id: I46b62c8c4aed7a1c9522d60db7f726ebcb30281c
Merge "Login with 'centos' user for node launch"