Bug 8351: Enforce check-style rules for restconf - sal-rest-docgen

    Organize Imports for Checkstyle compliance.
    Checkstyle compliance: line length.
    Checkstyle compliance: various types of small changes.
    Checkstyle compliant Exception handling.
    Checkstyle final clean up & enforcement.
    Add the fail on violation flag into the pom.xml .

Change-Id: If4a4a3b17f8a0b93433fb41813f2560b334fd45c
Signed-off-by: matus.kubica <matus.kubica@pantheon.tech>