Organize startup scripts by type

DRY up the init script case statement and make it more apparent which
style is used for each distro.
Use systemd startup scripts with EL7
Allow for distro/version specific assets

$asset_dir is set to the most specific asset dir available. For example,
if $linux is set to 'centos/6' and that dir exists, it will be used
instead of the dir 'centos' for package source files. This allows us to
drive the package config based on the distro+version if version specific
customizations are desired.
New post* scripts for EL7 to handle systemd

New post* scripts to handle systemd related tasks on EL7.
Merge branch 'master' into el7

Conflicts:
	build_mesos
Fix scripts/asset_dir reference

Fix a problem introduced by the merge. $scripts should now be
$asset_dir. The new variable was introduced with: 3c60363
Merge pull request #23 from mesosphere/el7

Updates to handle EL7 builds
New features to support distro/version specific customizations