fix "invalid sasl mechanism" error occuring in version 1.3.0

[saurabhshendye]

Getting the following error when installing version 1.3.0 with helm

time="2021-04-20T05:32:27Z" level=info msg="Starting kafka_exporter (version=, branch=, revision=)" source="kafka_exporter.go:517"
time="2021-04-20T05:32:27Z" level=info msg="Build context (go=go1.16, user=, date=)" source="kafka_exporter.go:518"
time="2021-04-20T05:32:27Z" level=fatal msg="invalid sasl mechanism \"\": can only be \"scram-sha256\", \"scram-sha512\" or \"plain\"" source="kafka_exporter.go:140"

This PR fixes this issue by adding support for specifying sasl.mechanism when installing this with helm chart.

[jeessy2]

I have the some error

[danielqsj]

thanks ! @saurabhshendye