Standardise asset compilation with RPM macros

Here's a small number of tidyups - net 41 lines removed :)

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 by luck.

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) Some hairy asset precompilation steps were 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.

(If the %%%% gives you a headache, look at the installed files from the scratch builds!)

3) Converted existing plugins using precompilation to use the macros and added it to foreman_docker to fix the issue uncovered in (1).

bastion:
http://koji.katello.org/koji/taskinfo?taskID=175375 el6
http://koji.katello.org/koji/taskinfo?taskID=175385 el7
http://koji.katello.org/koji/taskinfo?taskID=175393 f19

docker:
http://koji.katello.org/koji/taskinfo?taskID=175376 el6
http://koji.katello.org/koji/taskinfo?taskID=175387 el7
http://koji.katello.org/koji/taskinfo?taskID=175394 f19

staypuft:
http://koji.katello.org/koji/taskinfo?taskID=175378 el6
el7 & f19 not supported :(
The BRs are needed to be able to load the plugin in the build environment, as its gemspec dependencies are needed.

I needed to rebase the PR against the current version, as its dependency versions had changed, which is now done.