Allow support to modify the vmagent remoteWrite.maxDiskUsagePerURL setting

PMM documentation states that when using the ENV variables it is possible to change a few settings:

https://docs.percona.com/percona-monitoring-and-management/details/victoria-metrics.html#environment-variables

However, this setting is not listed:

https://docs.victoriametrics.com/#list-of-command-line-flags

Proposed solution: Add support to it.