Servlet api unbundle

GEOS-4014
=========

Fix servlet-api jar file from being included in .war builds by setting the javax.servlet dependency to be <scope>provided</scope> in the main pom.xml file.

Modules that make reference to the servlet-api have been fixed to include an un-versioned dependency in their own pom.xml files to allow them to compile

Tested by:
1) Compiling a release: mvn install -Drelease
  i) unit tests pass and build is successful
  ii) inspect generated .war file - servlet jar file is not present (jar tf web/app/target/geoserver.war |grep serv)

2) Compiling all extensions/community modules:  mvn clean package -P allExtensions -DskipTests=true - compiles fine.  Skipped the unit tests as some are failing anyway

3) deploy .war file to tomcat instance.  Look for the validateJarFile message in logs (grep -i validatejar *  -R) - not found