feat(slack pipeline): read secrets from env or volumeMount

[kirederik]

the current slack image uses kubectl to read a secret with a known name to find out the channel hook to send the notification to;

a better approach would to support reading the secret from env or from a file within the local system; this would allow penny to use env or volumeMounts on her pipeline spec. This would also making reading a secret with a known name redundant, so we can remove it.

tasks

• support for getting the slack hook url from a environment variable
• support for getting the slack hook url from a volumeMount
• drop support from reading from a known secret
• update docs
• bump the version and release a new image