add module with default nightlies environment

Here is how the conda environment currently looks like:

>>> from pprint import pprint
>>> from lhcbnightlyconf.env import environment
>>> pprint(environment)
{'channels': ['conda-forge', 'defaults'],
 'dependencies': ['python=3.7',
                  'cmake=3.18.4',
                  'ninja=1.10.1',
                  'ccache=3.7.12',
                  'pip=20.2.4',
                  {'pip': ['certifi==2020.11.8',
                           'chardet==3.0.4',
                           'cloudant==2.14.0',
                           'gitdb==4.0.5',
                           'gitpython==3.1.11',
                           'idna==2.10',
                           'git+https://gitlab.cern.ch/lhcb-core/nightly-builds/lb-nightly-configuration.git@master',
                           'git+https://gitlab.cern.ch/lhcb-core/nightly-builds/lb-nightly-db.git@master',
                           'git+https://gitlab.cern.ch/lhcb-core/nightly-builds/lb-nightly-functions.git@master',
                           'git+https://gitlab.cern.ch/lhcb-core/nightly-builds/lb-nightly-utils.git@master',
                           'python-gitlab==2.5.0',
                           'pyyaml==5.3.1',
                           'requests==2.24.0',
                           'semver==2.13.0',
                           'smmap==3.0.4',
                           'spython==0.0.85',
                           'tabulate==0.8.7',
                           'urllib3==1.25.11']}]}

This MR also includes helper function get_environment which enables to get the default environment with optional overrides, which may be added to the slot.metadata["environment"].