Fix SSLContextTest.test_SSLContext_defaultConfiguration failure

This test was failing because it assumed that all SSLContext
instances have the same set of TLS protocol versions enabled.

The fix refactored SSLDefaultConfigurationAsserts class into
SSLConfigurationAsserts class. The main difference is that the new
class has wider scope: it can assert that (1) the default
configuration of TLS/SSL primitives is as expected -- exactly what
the old SSLDefaultConfigurationAsserts class offered, and (2) that
TLS/SSL primitives are configured the same as a provided SSLContext.
Assertions about the default configuration of primitives other than
SSLContext are now implemented by asserting that these primitives are
configured exactly like the default SSLContext.

Change-Id: I52d6514768c4053054df2cf79e7182d8fd87bfe2