Cassandra DB plugin: Allow special chars in usernames

[pcman312]

Allows special characters in usernames by updating the default change-password statement to include quotes around the username.

In the meantime, this can be worked around by specifying the root_rotation_statements field on the database config:

$ vault write database/config/cassandra \
    plugin_name=cassandra-database-plugin \
    hosts=127.0.0.1 \
    protocol_version=3 \
    username="vault-admin" \
    password="myreallysecurepassword" \
    allowed_roles="*" \
    root_rotation_statements="ALTER USER '{{username}}' WITH PASSWORD '{{password}}'"

Testing

Running this with an automated test is a bit tricky and the change is small enough I'm comfortable with some manual testing:

Before fix:

$ vault write database/config/cassandra \
    plugin_name=cassandra-database-plugin \
    hosts=127.0.0.1 \
    protocol_version=3 \
    username="vault-admin" \
    password="myreallysecurepassword"

$ vault write -force database/rotate-root/cassandra
Error writing data to database/rotate-root/cassandra: Error making API request.

URL: PUT http://127.0.0.1:8200/v1/database/rotate-root/cassandra
Code: 500. Errors:

* 1 error occurred:
	* failed to update user: 1 error occurred:
	* line 1:16 mismatched input '-' expecting EOF (ALTER USER vault[-]...)

After fix:

$ vault write database/config/cassandra \
    plugin_name=cassandra-database-plugin \
    hosts=127.0.0.1 \
    protocol_version=3 \
    username="vault-admin" \
    password="myreallysecurepassword" \
    allowed_roles="*"

$ vault write -force database/rotate-root/cassandra
Success! Data written to: database/rotate-root/cassandra

[Valarissa]

Should we include a docs change to address the workaround for root credential rotation?

[calvn]

Can you add a changelog for this and milestone it under the appropriate release? Should this be a backport for 1.7.1?

[pcman312]

@calvn Done.

[pcman312]

@Valarissa re: docs: #11378