have a conservative default for keystore overwrite

[jalvz]

• use wording more suitable for apm-server
• do not overwrite keystore by default

refs: elastic/apm-server#2029, elastic/apm-server#2026, elastic/apm-server#2030

[jalvz]

thanks! test failure in metricbeat seems unrelated, ok to merge?

13:19:42 2019/03/20 13:09:47 httptest.Server blocked in Close after 5 seconds, waiting for connections:
13:19:42   *net.TCPConn 0xc42000e018 127.0.0.1:44700 in state active
13:19:42 SIGQUIT: quit
13:19:42 PC=0x489a11 m=0 sigcode=0
13:19:42 
13:19:42 goroutine 0 [idle]:
13:19:42 runtime.futex(0xcb6b88, 0x0, 0x0, 0x0, 0x0, 0x460962, 0xc42002e540, 0x0, 0x0, 0x7fff620d4e98, ...)
13:19:42 	/usr/local/go/src/runtime/sys_linux_amd64.s:530 +0x21
13:19:42 runtime.futexsleep(0xcb6b88, 0x0, 0xffffffffffffffff)
13:19:42 	/usr/local/go/src/runtime/os_linux.go:45 +0x4b
13:19:42 runtime.notesleep(0xcb6b88)
13:19:42 	/usr/local/go/src/runtime/lock_futex.go:151 +0xc7
13:19:42 runtime.stopm()
13:19:42 	/usr/local/go/src/runtime/proc.go:1952 +0xe5
13:19:42 runtime.findrunnable(0xc420032f00, 0x0)
13:19:42 	/usr/local/go/src/runtime/proc.go:2415 +0x2c8
13:19:42 runtime.schedule()
13:19:42 	/usr/local/go/src/runtime/proc.go:2541 +0x151
13:19:42 runtime.park_m(0xc420188300)
13:19:42 	/usr/local/go/src/runtime/proc.go:2604 +0x81
13:19:42 runtime.mcall(0x0)
13:19:42 	/usr/local/go/src/runtime/asm_amd64.s:351 +0x5b

[ph]

I ran into the same issue this morning, I've created #11336