allow user defined mounts for kinds and nodes

[hellt]

closes #88

this PR allows users to define Bind Mounts for nodes or kinds. For example:

topology:
  kinds:
    srl:
      type: ixr6
      image: srlinux
      license: license.key
      binds:
        - "/tmp/1test:/tmp/bla:ro"

or

topology:
  nodes:
    srl:
      kind: srl
      binds:
        - "/tmp/1test:/tmp/bla"

The format of bindmount string follows the format of docker/oci.

[karimra]

lgtm

[hellt]

merging
docs will be added in #119