Bump version to 0.15.1
Fix a typo in the program name
Remove support for OS API version 5, 10 and 15

snf-image wasn't working correctly with API versions prior to 20 anyway.
Source the variant file in the verify script

If the variant file has defined mandatory input variables like IMG_ID
the verification script should not fail to find them.

This fixes #35
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.
docs: Reflect the changes made in the hotfix

* Remove obsolete parts from the /etc/default/snf-image sample file
  shown in the configuration section.
* Update the documentation about having made the img_passwd an
  optional parameter.
Update changlog and version files for ver 0.15.1