API: Adding mapping between host and action parameters and using username field

Using username field when specified. When both password and rootPassword
are specified, prefer the rootPassword for backward compatibility.
Using HostMapper for mapping action and host objects to operation's parameters
in update, install, approve host actions.

Change-Id: Ifc7f6ba85ed6a5a790b3cc0acce3f5d6eb305789
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>