Only load required software definitions

[sethvargo]

We often have features in software dependencies that cause builds to fail, but those softwares aren't actually part of the build. For example, if you create a software definition like:

raise Exception

and commit to omnibus-software on master, all builds will fail. Omnibus should only load the required software definitions and leave the other ones alone. This will reduce the number of deprecation warnings and errors like #202.

[lamont-granquist]

+1

[lamont-granquist]

also only load the project definition for the project that you're actually building.

[sethvargo]

Started some work on #264