don't have sylvactl watch all namespaces by default, adopt -A/-n convention

Today sylvactl watch looks at all namespaces which results in a confusing output, and does not match the typically relevant use case (people usually look at a given cluster, not at all sylva-units Helm releases at the same time).

Proposal (for watch and inventory commands):

• don't have sylvactl watch all namespaces by default
• look by default at the current namespace
• keep the -n <n> parameter which allows to look at a specific one
• add a -A/--all-namespaces parameter to let users look at all namespaces