python-common: multi-line for yaml

python-common: multi-line for yaml

Currently when running the command ceph orch ls --export
multi-line strings are processed using default interpretation
meaning they could be printed with quotations instead of
the expected | . This causes a hassle for users who want to
copy their specifications to new yaml files as they have to
manually modify the yaml to remove the quotes and add | .
My code fixes this by modifying the yaml representer to
check for multi-line strings and wrapping them in a
YamlLiteralString class which will mark the multi-line string
to be processed with | . I've also added unit tests.

Signed-off-by: Timothy Q Nguyen <timqn22@gmail.com>