all: support `%{ENV_VAR}` placeholders in yaml configs in all the vm* components

deployment/docker: update Go builder from Go1.14.7 to Go1.15.0

app: respect CPU limits set via cgroups

Change deb pkg maintainer; fix deb prerm comment

Use getopt and add usage message to package/package_deb.sh

app/vmselect: reduce memory usage when exporting time series with big number of samples via `/api/v1/export` if `max_rows_per_line` is set to non-zero value