Fix Jenkins build error - "Unable to access URL brooklyn.webconsole.security.keystore.url"

[neykov]

Fix transformation of URL to file path - a good example why one shouldn't use string operations when converting between file path and URL.
Sample failure - https://builds.apache.org/job/incubator-brooklyn-pull-requests/1267/console

erifyHttps(brooklyn.launcher.BrooklynWebServerTest)  Time elapsed: 0.018 sec  <<< FAILURE!
java.lang.IllegalArgumentException: Unable to access URL brooklyn.webconsole.security.keystore.url: /home/jenkins/jenkins-slave/workspace/incubator-brooklyn-pull-requests%402/usage/launcher/target/test-classes/server.ks
    at brooklyn.util.ResourceUtils.getResourceFromUrl(ResourceUtils.java:287)
    at brooklyn.util.ResourceUtils.checkUrlExists(ResourceUtils.java:484)
    at brooklyn.launcher.BrooklynWebServer.getLocalKeyStorePath(BrooklynWebServer.java:515)
    at brooklyn.launcher.BrooklynWebServer.createContextFactory(BrooklynWebServer.java:443)
    at brooklyn.launcher.BrooklynWebServer.start(BrooklynWebServer.java:373)
    at brooklyn.launcher.BrooklynWebServerTest.verifyHttps(BrooklynWebServerTest.java:115)

[asfbot]

incubator-brooklyn-pull-requests #1272 SUCCESS
This pull request looks good

[sjcorbett]

Looks good.