Plan metadata to add VM creation parameters

[uakfdotb]

See #66 and #67.

Plans will have associated metadata, which will be arbitrary key-value pairs. These will be passed to the VM interface, which can look at the metadata to adjust the VM creation (and possibly other actions).

Schema:

CREATE TABLE plan_metadata (
    id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
    plan_id INT NOT NULL,
    k VARCHAR(64) NOT NULL,
    v VARCHAR(256) NOT NULL,
    KEY (plan_id)
);

Frontend will allow (un)setting metadata from the plan details page.

[uakfdotb]

For Vultr, you can now set these metadata keys:

• private_networking
• ipv6
• auto_backups
• dont_notify_on_activate

Set to "yes" to enable, "no" to disable. By default private networking and IPv6 are enabled, automatic backups and no notification are disabled.