Updates dummyTrustManager to use more supported security protocol

[scottopell]

Some customers run JMXFetch on a host install where their own JVM is used. These JVMs may restrict the available security algorithms to exclude SSL which would lead to an error similar to this:

java.security.NoSuchAlgorithmException: SSL SSLContext not available
   at java.base/sun.security.jca.GetInstance.getInstance(GetInstance.java:159)
   at java.base/javax.net.ssl.SSLContext.getInstance(SSLContext.java:168)
   at org.datadog.jmxfetch.HttpClient.<init>(HttpClient.java:91)
   at org.datadog.jmxfetch.App.<init>(App.java:115)
   at org.datadog.jmxfetch.JmxFetch.main(JmxFetch.java:56)
   at org.datadog.jmxfetch.App.main(App.java:91)

This PR changes our dummyTrustManager to use TLS instead of SSL (allowing the JVM to pick the specific version of TLS supported), which should have wider support across JVMs.

Also adds an error log specifying the impact of a missing dummyTrustManager to help with future triage.