Support for Ubuntu 18.04

[gnewson]

Resolves #249

st2mistral role has an added conditional so it doesn't run when on Ubuntu18.04. This is also applied to the postgresql role dependency.

Ubuntu18.04 added to the virtual machine list in the vagrant file and a small update to the readme to note that Ubuntu 18.04 is available as vagrant box.

[arm4b]

Thanks for the contribution!
@amanda11 Assuming your experience with adding EL8, can you please help reviewing this PR?

[nmaludy]

Isn't mistral deprecated and not included in Ubuntu 18 because it is python 3 only (mistral doesn't support python3).

[amanda11]

@armab yes I will review

[amanda11]

@nmaludy I think that is what changes does - it's not installing mistral or postgres when Ubuntu 18.
Not reviewed thoroughly yet but the stackstorm.yml is omitting those roles and therefore their install by the looks

[amanda11]

Thanks for the contribution.

Changes look good so far, I haven't checked the automated install script to check if any particular Ubuntu 18 v 16 script differences yet - in case any library changes for example. There were differences for redhat for python 3 that might also affect bionic - so it would be worth checking that (unless you have already).

However a few extra files to change. The.travis.yml controls what integrations are run and we should add Ubuntu 18 to this. Also the main/meta.yml and the meta.yml in each of the roles list the supported versions and need updating for bionic.

[amanda11]

I think there are a few more places to update, eg supported platforms, manual instructions, and list of deployments need to update

[gnewson]

I'll investigate and make those changes.

[arm4b]

Looks good! 👍

Thanks @gnewson for the contribution and @amanda11 for review and pointers.