ceph-dev-pipeline: extract matrix stage script bodies into helper funcs

ceph-dev-pipeline: extract matrix stage script bodies into helper funcs

Problem: We were hitting JVM's 64KB bytecode limit when adding new
distros to the matrix stage script.

Solution: Add helper functions for each stage in the matrix stage
script, such that the original block is significantly reduced, thus
ensuring we do not hit the JVM's 64KB limit again. This will also ensure
we can keep extending the matrix with more distros without worrying
about this issue.
Signed-off-by: Shraddha Agrawal <shraddha.agrawal000@gmail.com>