Support rpk cluster config set param=value

[patrickangeles]

Currently the syntax goes:

rpk cluster config set <parameter> <value>

This doesn't work for when the value is a negative number, because of how POSIX flags commonly work.

rpk cluster config set delete_retention_ms -1
Error: unknown shorthand flag: '1' in -1

The following does work:

rpk cluster config set -- delete_retention_ms -1

We can also propose an alternative that is more intuitive:

rpk cluster config set delete_retention_ms=-1