Modularise build scripts

Redesign the Deimos, Phobos, and Proxygen build scripts to make it
easier to add support for new OSs. OS-dependent requirements are
executed as functions to be implemented in sourced OS-specific scripts

Change log:
* replace Centos7 specific code with function calls
* pull implementations of centos7 build modules into sourced script
1 job for 3-modularise-build-scripts