Added option to force pull docker image.

Review: https://reviews.apache.org/r/28190
Fixed a few grammar mistakes in mesos.proto.
Silenced symbolic link to pre-commit in bootstrap.

Fixes MESOS-1960.

Review: https://reviews.apache.org/r/28303
Added validation for Resource objects with DiskInfo.

Review: https://reviews.apache.org/r/28258
Allowed C++ Resources to handle DiskInfo.

Review: https://reviews.apache.org/r/28264
Fixed a bug due to missing canonicalize.

Review: https://reviews.apache.org/r/28304
Added missed changes to DiskInfo patches..
Made libprocess generate valid HTTP.

While working on a pure Scala driver for Mesos, I discovered that
libprocess does not generate valid HTTP. It uses chunked encoding
combined with HTTP 1.0, which has only been added in HTTP
1.1. Additionally it stores the PID in the User-Agent field with an
incompatibe format ('@' and ':' are not allowed). This patch sets the
HTTP version to 1.1, adds an empty Host header (Host is mandatory in
1.1) and adds the Libprocess-From header. The User-Agent header is
left untouched for backwards compatibility reasons. Even strict HTTP
parser shouldn't error, but just ignore everything between the invalid
character and the CRLF.

Review: https://reviews.apache.org/r/27865
Allow mesos containerizer to destroy correctly at each state.