The BaseStrategy interface was moved to another location but the
documentation wasn't updated accordingly, so here I updated this
import path.
As an extra, I also updated the description of the Solution which
also had an outdated import path.
Closes-Bug: #1522034
Change-Id: I8cd43e84db6945391db06ec6dfe82ce02eccc7f4
Following https://review.openstack.org/#/c/245903, this patchset
documents the implementation and integration of custom Strategies
with Watcher.
Change-Id: I573758695d67d523834f145824b6f6cfdd8997fe