Make decommissioning timeout configurable

Recently I've noticed a number of integration tests failures due to
decommisioning timeout. Example:

https://savanna.mirantis.com/logs/15/71015/6/check/   \
gate-savanna-integration-heat-vanilla/44cfcad/console.html

I suggest to increase timeout to the default keystone token lifetime.
After the token is expired, we can not continue scaling anyway. On the
other hand, I see no value in setting it lower.

I've refactored code so that it counts time, not attempts to check
if decommissioning is ended. The later can give rather big timeout
deviation in case check operation takes some time to execute.

Closes-Bug: #1284170
Change-Id: Ibe30ea4d9c5b15ba8a9fa09d879a7852d3d2fad0
Merge "Make decommissioning timeout configurable"