add ability to detect container engine

[ofrobots]

Needed for https://github.com/GoogleCloudPlatform/google-cloud-node/issues/2125.

[stephenplusplus]

Just some quick notes

• can you make sure everything is kept in alphabetical order? vars/fns
• please use function ( as opposed to function( in all places

[stephenplusplus]

• no res needed in the new function

• missing semicolon

(can't comment by line, getting a GH error)

[ofrobots]

Addressed comments. PTAL.

[stephenplusplus]

Published under 0.6.0