Make img_passwd os parameter optional

If the parameter is not defined then the ChangePassword task will not
run at all.

This resolves #34
Fix a typo in HYPERVISOR variable substitution

Many thanks to Brian Candler https://github.com/candlerb for pointing
this out. This fixes #39
Give priority to command line os parameters

OS Parameters defined as command-line arguments should overwrite the
ones given in the variants file.

Many thanks to Brian Candler https://github.com/candlerb for the
suggestion and all the help.