[Impact]  * Snaps need to be able to specify if they require devmode or if they can be run confined. This will allow for snapd to provide reasonable errors if one tries to install a snap that cannot run successfully under confinement.  * The YAML property should be called "confinement," and it should have two options: "devmode" and "strict." It should be optional, and `snapcraft init` should set it to "devmode."  * The "confinement" YAML property should be copied into the resulting `snap.yaml`. * Example YAML: name: foo version: 1 summary: foo description: foo confinement: devmode parts: foo: plugin: nil [Test Case]  * Run `snapcraft init`. Make sure "confinement" is "devmode."  * Create a valid snapcraft.yaml and run `snapcraft` on it. Make sure the "confinement" value gets copied to the `snap.yaml`.  * Create a valid snapcraft.yaml and remove the "confinement" property. Run `snapcraft`. It should print a hint about defaulting to "strict", and "confinement: strict" should be in the resulting `snap.yaml`. [Regression Potential]  * snapcraft.yaml validation could be incorrect (required properties may not be required correctly, etc.)