Refactor shell script syntax and consistency

- Add a missing space for styling
- Replace legacy "\`...\`" with `$(...)`
- Quote variable to prevent word splitting
- Use builtin `command -v` instead of non-standard `which`
- Add two missing `>&2` redirection for error/warning message

Allow multiple YAML files to define zones

DynProvider dynamic serve_count=1 to match Route53

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

We hit this bug internally. It generally needed to be fix and was a larger
potential problem than expected since it was assuming TD assocaitions rather
than looking at them directly. Test changes were a little involved to suss
out, otherwise this is a fairly clean and simple fix.

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Might be possible/make sense to allow it to be configured later, but for now
Route53 doesn't support it so we'll go with 1 to make sure that things match
up behavior-wise.

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

The SplitYamlProvider itself now requires a directory matching the
zone name under its directory to contain all YAML files. This doesn't
actually change the intended usage at all, just how the configuration
file is laid out.

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

SplitYamlProvider extends and behaves similarly to YamlProvider, but
organizes the zone in multiple files by record, insteat of in a
monolithic YAML file.

YamlProvider has been slightly modified to make its extension easier.

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Normalize MX exchange to lower-case, fix #322

AzureProvider: update for azure-mgmt-dns 2.1.0 and add support for CAA records.

Route53Provider dynamic record support

Note that you can't specify a Host header for these which I believe will
complicate the ability to use this. Figuring that out will have to wait
until I or someone else has a use case for these...

This will ensure that deletes come before creates which are before upserts and
that records that uses aliases always come after their target (though implicitly
based on sorting types and not explicitly by looking at them.)

DynProvider only SUPPORTS_DYNAMIC if traffic_directors_enabled