Use the system keystore by default

Default system keystore is in the following file:
$JAVA_HOME/jre/lib/security/cacerts.

Custom keystore can be set either in /etc/sysconfig/gitbucket by
specifying the following option:
GITBUCKET_JVM_OPTS="-Djavax.net.ssl.trustStore=/path/to/the/cacerts"
or in Gitbucket's System Settings.