force newline during authorized_keys file build

[jalleyne]

Fixes issue #283 with missing new line characters between authorization keys if the file does not already contain these new lines. this is usually an easy fix by appending a new line to the file and mounting as described in the README, but in situations like helm charts the values being passed in are stripped of whitespaces making this difficult to solve.

[v0l]

Please merge this @atmoz