Factor out RemoteExecutor from SSHExecutor.

Summary:
A lot of the pieces of SSHExecutor can be shared with my AdbExecutor
(https://android-review.googlesource.com/#/c/138807/).

Reviewers: EricWF, jroelofs

Reviewed By: jroelofs

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8117

llvm-svn: 231815