A way to "pre-configure" limits for virtual hosts

[michaelklishin]

At the moment virtual host limits can only be applied to existing virtual hosts. It would be
useful to allow for pre-defined limits. With a feature like this, it would be realistic
for RabbitMQ-as-a-Service environments to allow the user to create more virtual hosts while still
enforcing some limits.

Suggested by @illotum.

What I have in mind is something like this:

default_limits.vhosts.1.pattern = ^device
default_limits.vhosts.1.max_connections = 10
default_limits.vhosts.1.max_queues = 10

default_limits.vhosts.2.pattern = ^system
default_limits.vhosts.2.max_connections = 100
default_limits.vhosts.2.max_queues = -1

default_limits.vhosts.3.pattern = .*
default_limits.vhosts.3.max_connections = 20
default_limits.vhosts.3.max_queues = 20

defines three groups of limits that match virtual hosts by name. The first group to match will
be used to apply the limits right after the matching virtual host is declared.

[michaelklishin]

Done in #6172 and #6191.