Read-replica support

[smlx]

Let's try this again.

Add read replica support to the provisioner. This time the change is not breaking - if the readreplica secret is missing, then:

• the readreplica service will not be provisioned in the target namespace, but the regular service will be.
• the DB_READREPLICA_HOST field in the secret will be empty.

Also update README and add notes on development environment, along with scripts and a basic test suite.

[steveworley]

It looks like we'll be adding a single host to the mariadb_readreplica_hostname, are we expecting that we will only surface one replica via this environment variable? If we had access to multiple readers how would this come back from the DBaaS controller?

For example - Drupal can be configured with n number of replicas and it will send queries to each in a round-robin format. The configuration we'd be looking to use would look like

$databases['default']['replica'][] = [
  'driver' => 'mysql',
  'database' => 'drupaldb1',
  'username' => 'username',
  'password' => 'secret',
  'host' => 'dbserver1',
];

[smlx]

Drupal can be configured with n number of replicas and it will send queries to each in a round-robin format.

Very interesting, I didn't know that!

You're right, this PR doesn't address the multiple-read-replica scenario. I don't think it's worth bundling that into this PR because it's big enough as is. But maybe it makes sense to add a numerical suffix to the environment variable so that multiple read-replicas can be added in future.

e.g. make the variable in this PR DB_READREPLICA_HOST_0, then in future when multiple read-replica support is added we can have DB_READREPLICA_HOST_1, DB_READREPLICA_HOST_2 etc.

[smlx]

...or better, use DB_READREPLICA_HOSTS, and make it a list of space-separated hostnames.

[smlx]

This is ready for review, but please don't merge/deploy until next week to avoid any issues over the weekend.

[smlx]

This is now ready for review.

[thom8]

LGTM

[shreddedbacon]

LGTM