We should simplify enabling text file collector extensions for PMM user

We should enable it by default and create specific directory which is part
of the agent install where users simply can put text files from the scripts
they create

–
Peter Zaitsev, CEO, Percona

TODO
1) Implement automatic collection of *.prom text files from /usr/local/percona/pmm-client/textfile-collector by default making change in https://github.com/percona/node_exporter/blob/master/collector/textfile.go#L36
2) create /usr/local/percona/pmm-client/textfile-collector during installation of pmm-client
3) place to /usr/local/percona/pmm-client/textfile-collector file sample.prom with commented example and link to
https://github.com/prometheus/node_exporter#textfile-collector for usage and examples
4) ? add some tests to ensure this feature will work later