schema cli: Add schema subcommand to cloud-init cli and cc_runcmd schema

This branch does a few things:
  - Add 'schema' subcommand to cloud-init CLI for validating
    cloud-config files against strict module jsonschema definitions
  - Add --annotate parameter to 'cloud-init schema' to annotate
    existing cloud-config file content with validation errors
  - Add jsonschema definition to cc_runcmd
  - Add unit test coverage for cc_runcmd
  - Update CLI capabilities documentation

This branch only imports development (and analyze) subparsers when the
specific subcommand is provided on the CLI to avoid adding costly unused
file imports during cloud-init system boot.

The schema command allows a person to quickly validate a cloud-config text
file against cloud-init's known module schemas to avoid costly roundtrips
deploying instances in their cloud of choice. As of this branch, only
cc_ntp and cc_runcmd cloud-config modules define schemas. Schema
validation will ignore all undefined config keys until all modules define
a strict schema.

To perform validation of runcmd and ntp sections of a cloud-config file:
$ cat > cloud.cfg <<EOF
runcmd: bogus
EOF
$ python -m cloudinit.cmd.main schema --config-file cloud.cfg

$ python -m cloudinit.cmd.main schema --config-file cloud.cfg \
  --annotate

Once jsonschema is defined for all ~55 cc modules, we will move this
schema subcommand up as a proper subcommand of the cloud-init CLI.

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