Standardise asset compilation with RPM macros

Here's a small number of tidyups.

1) Removed some runtime deps (sass, jquery) from Foreman, which are only needed for precompilation.  They're already on foreman-assets.  I realised that foreman_docker had been relying on runtime asset compilation.

http://koji.katello.org/koji/taskinfo?taskID=175330 f19
http://koji.katello.org/koji/taskinfo?taskID=175332 el6
http://koji.katello.org/koji/taskinfo?taskID=175334 el7

2) Asset precompilation was being copied from spec file to spec file, so it's now a simple RPM macro provided by foreman-assets, making it a one-liner to compile assets.  Some other standard macros (paths, commands) are added to foreman, but I'm not relying on them across general plugins yet.

3) Converted existing plugins using precompilation to use the macros and added it to foreman_docker to fix the issue uncovered in (1).

(plugin scratches to follow, it's a bit tricky)