Reinstate Bitronix's default server ID, provide property to override it

Previously, Bitronix's server ID was hard-coded to be
spring-boot-jta-bitronix. This created the possibility of multiple
transaction managers performing recovery on each other's behalf as
they would be unable to identify their own XIDs due to the common
server ID.

This commit reinstates the default (which is the IP address of the
machine on which Bitronix is running), and introduces a new
property, spring.jta.transaction-manager-id, that can be used to
configure the id for both Atomikos and Bitronix. A cautionary note
has also been added to the documentation for Atomikos and Bitronix
explaining the need to configure this property.

Closes gh-1548