Add a new API to verify if a Deployable is deployed

Add a Deployer.isDeployed(Deployable) API.

Implementation notes:

• For local deployers we can assume a deployable is deployed if it's present in the container's deployment directory (i.e. in webapps/ for tomcat, deploy/ for JBoss, etc)
• For remote deployers we need to verify this can be implemented for JBoss (we already have it implemented for Tomcat) and other containers (today we only have remote deployers for JBoss and Tomcat ).

NOTE: Having this new API will allow us to refactor the Deployer.redeploy() implementation which can then be refactored and implemented generically in AbstractDeployer:

if isDeployed
  undeploy()
  deploy()
else
  deploy()
endif