cephadm: updates to cephadm configuration and runtime behaviour

cephadm: updates to cephadm configuration and runtime behaviour

Multiple changes based on PR feedback
- invocation of set-exporter-config now sets all paramters
- customvalidation of exporter-config now throws
argparse exceptions
- config validation now checks for SSL crt/key format
- failing threads are now caught and change the http
response to the caller (500, and 503 may not be returned)
- bootstrap options descriptions tidied up

Signed-off-by: Paul Cuzner <pcuzner@redhat.com>