DTLS certificate configuration

[ghjansen]

This issue is related to #254 and #255.

Cipher suite certificates are currently used as a internal resource, but maybe the location of the file should be configurable. Investigate and adapt as needed.

[hrosa]

The configuration file should adopt 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" algorithm="rsa">
        <certificate path="/path/to/certB.pem" key="/path/to/keyB.pem" algorithm="ecdsa">
    <certificates>
</dtls>

Keep in mind that suitable default values should be provided to load the sample RSA and ECDSA certificates. Make special mention in the docs that this implies a serious security risk and that users should provider their own certificates.

Also noticed how you're explicitly identifying the certificate's algorithm. This is useful when configuring this.

[ghjansen]

@hrosa the current implementation of DtlsSrtpServer works with multiple certificate resources and only one key resource, in a cardinality N -> 1. But the configuration suggested by this issue allows to configure multiple certificate resources, each one containing a specific key resource, forming a cardinality 1 -> 1. So, i'm not sure what is the desired final result in comparison with the current one. Do you mind to elaborate a little on how the new configuration should be used by DtlsSrtpServer? Thanks.