Allow specifying service env variables

[petrutlucian94]

We're adding a mechanism to specify custom service env variables:
/var/snap/k8s/common/args/${service_name}-env

Note that this feature is already supported by microk8s[1] and documented by k8s-dqlite[2].

[1] https://github.com/canonical/microk8s/blob/master/microk8s-resources/wrappers/run-k8s-dqlite-with-args#L38-L43
[2]

k8s-snap/k8s/lib.sh

Lines 184 to 194 in 4615c03

	k8s::common::execute_service() {
	service_name="$1"
	k8s::common::setup_env
	# Source arguments and substitute environment variables. Will fail if we cannot read the file.
	declare -a args="($(cat "${SNAP_COMMON}/args/${service_name}"))"
	set -xe
	exec "${SNAP}/bin/${service_name}" "${args[@]}"
	}