Make minimum and maximum version of DTLS configurable

[hrosa]

Make min and max protocol versions configurable according to the following format:

<dtls>
    <minVersion>1.0</minVersion>
    <maxVersion>1.2</maxVersion>
    <cipherSuite> cipherA,cipherB,...,cipherN</cipherSuite>
    <certificates>
        <certificate path="/path/to/certA.pem" key="/path/to/keyA.pem">
        <certificate path="/path/to/certB.pem" key="/path/to/keyB.pem">
    <certificates>
</dtls>