fix(schema): accept an integer netmask in network config v1

[sbraz]

Proposed Commit Message

fix(schema): accept an integer netmask in network config v1

A config drive setting an IPv6 route netmask as an integer produces this,
even though the route is applied correctly:
```
schema.py[WARNING]: network-config-v1 failed schema validation! You may run 'sudo cloud-init schema --system' to check the details.
```
```
# cloud-init schema --system
Error: Cloud config schema errors: config.0.subnets.1.routes.0.netmask: 128 is not of type 'string'
```

Nothing that reads the value requires a string:

- `_normalize_net_keys()` passes `netmask`, for subnets and routes, to
  `ipv4_mask_to_net_prefix()` or `ipv6_mask_to_net_prefix()`, which document
  accepting an integer or a string representation of one.
- `test_ipv6_static_routes_ip_cmd` renders a v1 config containing both
  `"netmask": 24` as an integer and `"netmask": "32"` as a string.

Accept an integer alongside a string, for subnets and routes, and reword the
description to cover IPv6.

Test Steps

# /tmp/net-v1.yaml
network:
  version: 1
  config:
    - type: physical
      name: eth0
      subnets:
        - type: static6
          address: "2001:db8::1/128"
          gateway: "2001:db8::2"
          routes:
            - network: "2001:db8:1::"
              netmask: 128
              gateway: "2001:db8::2"

Before:

$ cloud-init schema -t network-config -c /tmp/net-v1.yaml
Error: Cloud config schema errors: network.config.0.subnets.0.routes.0.netmask: 128 is not of type 'string'
Error: Invalid schema: network-config

After:

$ cloud-init schema -t network-config -c /tmp/net-v1.yaml
Valid schema /tmp/net-v1.yaml

Unit tests:

$ tox -e py3 -- tests/unittests/config/test_schema.py
156 passed, 2 skipped

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)