Added os::execvpe to stout.

Review: https://reviews.apache.org/r/24679
Updated subprocess to use os::execvpe.

Review: https://reviews.apache.org/r/24681
Updated mesos to use os::execvpe.

Review: https://reviews.apache.org/r/24682
Fixed the flaky SlaveRecoveryTest.ShutdownSlave.

Review: https://reviews.apache.org/r/24678