* Allow testnet hostnames to be overridden

This allows one to specify the `--hostname` flag multiple times, each
time providing an additional custom hostname for a respective peer
(validator or non-validator). This overrides any of the
`--hostname-prefix` or `--starting-ip-address` flags.

The string array approach is taken instead of the string slice approach
(see the pflag docs:
https://godoc.org/github.com/spf13/pflag#StringArray) because the string
slice approach (a comma-separated string) doesn't allow for cleaner
multi-line BASH scripts - where this feature is intended to be used.

* Reorder conditional for clarity with simpler earlier return

* Allow for specifying peer hostname suffix

* Quote values in help strings for greater clarity

* Fix command switch

* Add CHANGELOG_PENDING entry for PR

* Allow for unique monikers

The current approach to generating monikers for testnet nodes assigns
the local hostname of the machine on which the testnet config was
generated to all nodes. This results in the same moniker for each and
every node.

This commit makes use of the supplied `--hostname-prefix` and
`--hostname-suffix`, or `--hostname` parameters to generate unique
monikers for each node. Alternatively, another parameter
(`--random-monikers`) allows one to forcibly override all of the other
options with random hexadecimal strings.

* Update CHANGELOG_PENDING entry for new command line switch