* Allow tests to specify mysql connection info.

User can set MYSQL_TEST_USER, MYSQL_TEST_PASS and MYSQL_TEST_HOST
environment variables for test environments that need that.

* Changes from testing.

Need to set both time fields as '0000-00-00' depends on a feature
deprecated in MySQL 5.7.4.

Go lint wanted snake case for my sql_config var. sqlConfig it is.

* Go go idioms.

Based on feedback from Niall, a cleaner way to do this in Go.

Fix newly-broken tests

Simplify key generation

Use functions to build key generation options.
Make it entirely optional.

sqlite uses CGO which prevents the building of statically-linked binaries.
This change will omit sqlite support when building a static binary with:
CGO_ENABLED=0 go build --ldflags '-extldflags "-static"'

When configured the server will request a TLS certificate for the specified server name from LetsEncrypt

Avoid unmarshalling into an intermediate struct.
Better tests.

The 'datastore' string option is deprecated and will be removed in a
future version. The new 'database' map option is preferred.

Vault is supported for the following:
As a well-known filesystem for TLS cert, TLS key and SSH signing key.
For configuration secrets for cookie_secret, csrf_secret, oauth_client_id and oauth_client_secret options.