Microk8s: [installer] multipass VM name should be `microk8s-vm`

[ceefour]

As documented in https://microk8s.io/#get-started,
Getting Started suggests a VM name as microk8s-vm, instead of the currently used MicroK8sVM by installer.

While it's a subtle detail, it's good to avoid subtle issues regarding documentation etc. especially for Windows and MacOS users.

[ktsakalozos]

Thank you for spotting that @ceefour . You are right, we should be consistent.

[kwmonroe]

In addition to the VM name, i think it would also be good to synchronize the default constraints with the docs. Currently, microk8s multipass docs say this:

multipass launch --name microk8s-vm --mem 4G --disk 40G

But if users run one of the installers, they'll get these constraints by default:

https://github.com/ubuntu/microk8s/blob/master/installer/common/definitions.py#L23-L25

[ktsakalozos]

The VM naming is fixed https://github.com/ubuntu/microk8s/pull/1125

The default VM disk size is set to 50 GB in https://github.com/ubuntu/microk8s/pull/1112