Add build:image support using 'brew image-build'

This is the first crack at adding support for building image using 'brew image-build'.

In the future functionality may need to change as issues with 'brew image-build' are resolved:
- 1119813 - Access to images built with 'brew image-build' should not require manually opening a brewweb URL
    https://bugzilla.redhat.com/show_bug.cgi?id=1119813

- 1119825 - Allow the 'brew image-build' kickstart it to be passed on the command line for non-scratch builds.
    https://bugzilla.redhat.com/show_bug.cgi?id=1119825

- 1119835 - 'brew image-build' config file and command line arguments should be fully intermingle.
    https://bugzilla.redhat.com/show_bug.cgi?id=1119835

-  1119837 - 'brew image-build' should provide checksums for image file
    https://bugzilla.redhat.com/show_bug.cgi?id=1119837

We could investigate generating the config files on the fly and using the brew API.
The brew CLI had been used as instructions and examples for using 'brew image-build'
all used the CLI. The API solution was just introduced as a possibility in reply to 
bugzilla ticket https://bugzilla.redhat.com/show_bug.cgi?id=1119813.

So although functionality may change in the future this pull request represents
a major simplification of the CFME image build workflow.
@jvlcek We might want to move to using AwesomeSpawn instead of the `system` calls. Chances are, we might want to capture, stderr, stdout, exit status and AwesomeSpawn provides all of that plus sanitizing.