SSL: Restore default flags when reinitializing.
SSL: Restore default flags when reinitializing.

This moves the initialization of the SSL `Flags` into the `initialize`
method.

Prior to this, SSL flags would not be completely cleaned up between
tests.  Test cleanup unsets all environment variables related to SSL.
However, the flags object would persist some of these variables between
tests.
For example:
1) Test 1 unsets all flags, then sets flag `SSL_CA_FILE`.
2) Test 2 unsets all flags, then sets flag `SSL_CA_DIR`.
   `SSL_CA_FILE` will still be set.
3) Test 3 unsets all flags.  `SSL_CA_FILE` and `SSL_CA_DIR` are still
   set.

Review: https://reviews.apache.org/r/39495

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