Have generate-jwt accept named arguments instead of positional ones

[maxalbert]

Spinning out of a comment on PR #733:

Currently the generate-jwt CLI utility accepts positional arguments:

generate-jwt TEST_USER SECRET 1 TEST_SERVER --all-access http://localhost:9090

For readability (and to avoid accidentally mixing up the order), it would be nice to have these as named arguments instead:

generate-jwt --all-access \
    --username TEST_USER \
    --secret-key SECRET \
    --lifetime 1 \
    --flowapi-server-name TEST_SERVER \
    http://localhost:9090