Make yamllint config more strict

[tyll]

the truthy and document_start checks are currently disabled in yamllint by the system roles default template but they could be enabled for the network role

The truthy check also allows to specify other values, so allowing yes and no might be a better alternative than disabling it alltogether.

[pcahyna]

https://github.com/linux-system-roles/template/pull/34

[liangwen12year]

The truthy and document-start checks in the network role are enabled.