do not require VS when only using installer

[timriker]

Is your feature request related to a problem? Please describe.
When using only the installer portion of this plugin, it should not require a vs-list as it's not used during install.

Describe the solution you'd like
when using -a dns-rfc2136 and -i bigip I get the error:
--bigip-vs-list is required when using the F5 BIG-IP plugin

Describe alternatives you've considered
I'm using dns-rfc-2136 to request wildcard certificates.
I want to use this plugin to install them on multiple F5s that are in different groups.

Additional context
The F5s are in different datacenters and DNS load balanced between them.
The names of the virtual servers are different in each group.