dnsmasq-flags option does not support non-kv flags

currently "dnsmasq-flags" charm option only supports flags in key-value pair format, like

dhcp-option=42,0.0.0.0

and it works fine for such values, but there are some dnsmasq options that have no value, e.g.:

log-queries
log-dhcp
no-negcache
dhcp-authoritative
dhcp-rapid-commit

to name a few. When such options are supplied as dnsmasq-flags, the rendered dnsmasq.conf file simply becomes invalid:

$ juju config neutron-gateway dnsmasq-flags="log-dhcp,dhcp-option=42,0.0.0.0,no-negcache,srv-host=_ldap._tcp.example.com,ldapserver.example.com,389"

# rendered /etc/neutron/dnsmasq.conf
log-dhcp,dhcp-option = 42,0.0.0.0,no-negcache
srv-host = _ldap._tcp.example.com,ldapserver.example.com,389

# expected /etc/neutron/dnsmasq.conf
log-dhcp
dhcp-option = 42,0.0.0.0
no-negcache
srv-host = _ldap._tcp.example.com,ldapserver.example.com,389

Expectation: We should be able to supply non-kv config flags to dnsmasq, and the resulting dnsmasq.conf file must be a valid dnsmasq configuration file.