Add config field to specify chroot mapping for exec driver

[parasyte]

• Same format as used by the internal chroot mapping
• Map: source_path -> dest_path
• Example HCL:

client {
  chroot_env {
    "/etc" = "/etc"
    "/lib" = "/lib"
    "/opt/projects/foo/bin" = "/usr/bin"
  }
}

[parasyte]

This can address #1507 by specifying a smaller subset of files and directories for your chroot.

[diptanu]

if len(e.command.ChrootEnv) > 0 since len of nil maps are 0

[parasyte]

@diptanu Addressed all review comments. I haven't been able to validate the new tests, because the test suite is failing on the point in master where I forked.

[dadgar]

Can you also add documentation to the website /docs/agent/config.html

[parasyte]

@dadgar 👍 Incoming.

[dadgar]

Thanks for the docs they look great. Though the test is failing: https://travis-ci.org/hashicorp/nomad/builds/150180275#L3656

[parasyte]

Yep, it's a bad test. I know alloc, dev, and proc are mounted into the chroot (forgot about those), but where do tmp and local come from? Also, I should really look into running individual tests to speed up the dev cycles.

[jshaw86]

@dadgar @diptanu what milestone will you all be bringing this PR into? Trying to decide if we need a build pipeline internally for a while till it lands into a milestone

[diptanu]

@parasyte tmp and local are created by the nomad client in every allocation directory.

[diptanu]

@jshaw86 I am hoping this would get into the 0.4.1 release if everything goes well.

[parasyte]

@dadgar The test should be fixed now!

@diptanu Thanks, I was able to find where the local directories were created.

[dadgar]

This test is also failing: https://travis-ci.org/hashicorp/nomad/builds/150759498#L5510

[parasyte]

Ok, got that one too!

[diptanu]

@parasyte Thanks for the contribution! This has landed on master now.

cc/ @jshaw86

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