[Metal] Plans do not match API spec

[ctreatma]

Copied from equinix-labs/metal-go#154

The API spec says that the type of a drive in a plan spec must be one of the following:

enum:
- HDD
- SSD
- NVME

However, the API can return the value NVMe, as mentioned in this PR comment from metal-cli: equinix/metal-cli#350 (comment)

The spec needs to be updated so that all possible values for type are documented accurately in the spec.

[ctreatma]

In addition to the enum mentioned in the issue description, there are also other enums in the plan spec that do not appear to match all possible plans returned by the Metal API

[ctreatma]

Fixed by #67 which removed the enums from the Plan schema and was included in v0.42.0.