boulder: Profile script keys should fall through to global keys if undefined in profile

[ReillyBrogan]

If the script keys environment, setup, build, install, check, or workload are not set in a profile but are set at the root of the yaml document then the build should use the "global" version for that profile.

emul32: yes
setup : |
    %configure
build  : |
    %make
install: |
    %make_install
profiles:
    - emul32:
        install :|
            %make_install
            rm %(installroot)/bad/file

The above should result in the x64 build running %configure, %make, and %make_install while the emul32 build runs %configure, %make, %make_install, and rm %(installroot)/bad/file. The current behavior is that the emul32 build only runs the install phase, skipping the setup and the build ones.

[ermo]

We need this yesterday.