Change default CNI network name, bridge name, and subnet

[stealthybox]

Fixes #424
Fixes #455

The solution in this patch for #424 is to use a static subnet in the 10.0.0.0/8 range.
This avoids the complexity of collaborating with the docker defaulting logic which requires calculating and iterating a over a similar list of subnets and checking to see if they're in use by the kernel.
This:

• keeps the code simple
• has the future side-effect of making issues more searchable via google when users submit issue details with the default network setup

Rationale for the subnet choice is documented on the constant.

Changing these CNI details will ultimately necessitate that any current users of our (mostly broken) default CNI setup will likely want to migrate.
This is not forced on users though. The config is still only written when the CNI net conf dir is empty.

[stealthybox]

cc @najeal -- pinging you as discussed
Please review at your leisure :)

[chanwit]

both changes

• ignite0
  and
• 10.61.0.0/16
  lgtm

[najeal]

Looks good to me 👍

[chanwit]

Thank you @najeal to help review this, and thank you @stealthybox for taking care of this.

[lucidprogrammer]

Thanks for this update, however, its possibly causing #462