migrate to poise python and Chef12+ syntax

[vytstank]

• Migrate python depends to poise-python
• Chnage node attribute access syntax to preferred in Chef12+
• Bump cookbook version to 0.4.0

Tested with vinted-exabgp 0.4.2:

• converge with Berksfile cookbook 'exabgp', git: 'git://github.com/vinted/exabgp-cookbook.git', tag: 'v0.3.0-vinted'
• second converge with cookbook 'exabgp', git: 'git://github.com/vinted/exabgp-cookbook.git', ref: '0ae588695ef8b6d18181a6f12d5058d7822bdc93'

[vytstank]

Sadly this breaks when python cookbook has already deployed pip > 10 into systemd python environment. Cookbook poise-python has python scripts to determine dependencies, which break during the run. poise-* cookbooks themselves have been archived in github by maintainer, which means no further development.

See
poise/poise-python#140
poise/poise-python#142

New deployments are OK.