Create a parent pom for md-sal sub-projects

This will contain common configurations for checkstyle and
findbugs (once all violations are fixed). Also, isolates the
config system dependencies in one place which will make it
easier to remove CSS.

Change-Id: Ifcf4bcd5e1b96483714bad51858264f511c21237
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>