[inputs.vsphere] Deprecate force_discover_on_init

[prydin]

Relevant telegraf.conf:

force_discover_on_init = false

System info:

Every version to date, every platform

Steps to reproduce:

1. Set force_discover_on_init = false in the config file.

Expected behavior:

The intended behavior was that a partial metric collection was going to start prior to the first discovery completed.

Actual behavior:

Several unintended and unpredictable effects. Most of all, the flag was kind of useless, since no metric collection could take place before the first discovery was completed even if it ran in the background. Furthermore, the -test flag didn't work with the force_discover_on_init was set to false.

[danielnelson]

The plugin may now produce some did not complete within its interval warnings on startup, but I plan to have parallel plugin startup implemented before the 1.14 release which will resolve this.