Task add working dir feature

[rybushkindmitry]

The working_dir parameter adds the ability to change default working directory for container.

When using TLS transport, SIPp will expect to have two files in the current directory: a certificate (cacert.pem) and a key (cakey.pem). If one is protected with a password, SIPp will ask for it.

Create a docker volume and place the certificate files in it.
Mount volume with certificates to the container and setup working_dir to that folder.
Specify in the args to use the TLS transport with options "-t l1".

tasks:
...
  - name: tls client
    type sipp
    config_file: invite.xml
    working_dir: /certs
    args: "-t l1"
    volumes:
      - sipp-certs
...

volumes:
  sipp-certs:
    bind: /certs
    mode: ro

[razvancrainea]

Awesome, thanks for the PR!