Fixes to image build process

[csegarragonz]

In this PR I fix a series of issues I have run into whilst trying to release a new docker image:

• Pin conan dependencies to a specific revision. Otherwise, the same package versions can fail in subsequent builds if the revision for the package version changes upstream (see Pin Conan dependencies to revision faabric#200).
• Use only virtual environments to handle python dependencies. Due to an issue with pip's editable installs, our python builds stopped working. A workaround seemed to be enabling site.ENABLE_USER_SITE in the package's setup.py (i.e. in faasmtools and faasmcli) but this derived in other problems. In the end I have decided to use only virtual environments around (as we do in regular development environments). Note that, to prevent a clash between container and out-of-container builds in the venv directory, I make out-of-container builds use venv-bm as their virtual environment root.
• Fix linking issue in aws-sdk-cpp. This conan package seems very unstable and fragile, we revert to installing it as an ExternalProject in CMake. I add an interface library + alias to make the replacement transparent.

Closes #324

[Shillaker]

Can we avoid this repetition with a custom shell or by using alias for inv at the start?

I think we can put bash as the default shell too.

[csegarragonz]

I set a default shell (that activates the virtual env. before running the command) only for the job that runs the tests.

Note that we can't set the shell as a default for all the workflow as not all jobs run in the same container (thus don't have the venv set, neither they need it).

[Shillaker]

Awesome, nice job sorting this all out 👍