res_pjsip: Add 'dtls_fingerprint' option to configure DTLS fingerprint hash.

During the latest update to DTLS-SRTP support the ability to configure
the hash used for fingerprints was added. This gave us two supported ones:
SHA-1 and SHA-256. The default was accordingly updated to SHA-256.
Unfortunately this configuration ability was not exposed within res_pjsip.
This change adds a dtls_fingerprint option that controls it.

#SIPit31
........

Merged revisions 424290 from http://svn.asterisk.org/svn/asterisk/branches/12
........

Merged revisions 424291 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@424292 65c4cc65-6c06-0410-ace0-fbb531ad65f3