generate a new TLS cert pair for btcsim

Removes dependency on local btcd cert pair and also fixes tests on travis.
I think the app dir (well, and the cert/key filepaths too) should be a global variable (and ones that change if/when a different app directory is set from a flag).  It would remove the error path here and we could generate these filepaths exactly once at startup.

Don't really care for getAppDir either due to the unnecessary stat.  It should be checking for and creating the app directory at startup when necessary.