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.
I had thought of this and I agreed 100% However I should have noted this in the PR summary, I didn't want to spend a lot of time on capturing and analyzing output from AwesomeSpawn until we learn how the brew CLI will support what we need or if we need to use brew API instead. Until those issues are resolved I think we should stick with the simple system calls.