BootstrapManager blocks in-place upgrades for cluster installations

In-place upgrades of Confluence Clustered installations doesn't work, and hasn't worked since Confluence 5.1.2.

To reproduce:

1. Setup 2 nodes of a Confluence 5.0.x clustered installation
2. Shutdown the nodes
3. Upgrade node 1 to Confluence 5.4, verify that the database is upgraded as expected
4. Upgrade node 2 also, note that startup fails due to "the build number in the home directory [4109] doesn't match the build number in the database [4723]"

Upgrading cluster installations to any version prior to 5.1.2 works fine, but upgrading any cluster installations to 5.1.2 or later will likely fail.

Importantly, this only affects Confluence clustered installations, and only those being upgraded from one version to another using in-place upgrades rather than site imports.