Use YAML format for Capstanfile

Switch to YAML format for Capstanfile which is more widely used and less
verbose than the INI file format.

An example Capstanfile looks as follows:

  base: osv-base

  cmdline: /tools/hello.so

  files:
    /tools/hello.so: hello.so

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Simplify BuildImage() error handling

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>