Adjust build jobs based on cores available.

Automatically set the number of build jobs (make -j) based on the
number of cores available. The optimal setting is generally 2x the
number of cores. If we set this to an arbitrarily large value, it
tends to degrade performance and may even exhaust available memory
on machines with less than 8GB of RAM.
Merge pull request #3 from mesosphere/wip-ncores-r1

Adjust build jobs based on cores available.
Merge branch 'master' into wip-jeremy
Ensure Mesos prereqs are available before startup

Delay Mesos startup to just after the standard sysv scripts, to
ensure that all of the Mesos prereqs are met (networking, zookeeper,
etc.) If we start at the beginning of a runlevel, Mesos may fail to
initialize due to the high count of failed respawn attempts by
upstart. This was particularly a problem on RHEL and CentOS.