Support executing pipeline on user provided infrastructure

Hi

Currently pipelines execute on Bitbucket provided infrastructure. This runs into multiple issues such as:

1. Inability to use existing fixtures/databases/services for code being tested.
2. Security limitations such as executing pipeline over instances with pre-assigned roles (currently this is handled via SECRET/ACCESS keys)
3. Copying large amounts of data as opposed to using a pre-existing container with pre-loaded data

The ability to deploy pipelines to user provided infrastructure should mitigate this! Please enable support for the same