EL7 :: thread_create failed: Operation not permitted

# Issue report

## What version of MicroCeph are you using ?
18.2.0+snap21acc74fff

## What are the steps to reproduce this issue ?

1. snap install microceph
2. snap refresh --hold microceph
3. microceph cluster bootstrap

## What happens (observed behaviour) ?
```
runtime/cgo: pthread_create failed: Operation not permitted
SIGABRT: abort
PC=0x7fb7f4c319fc m=0 sigcode=18446744073709551610

goroutine 0 gp=0x105df00 m=0 mp=0x105eb00 [idle]:
runtime: g 0 gp=0x105df00: unknown pc 0x7fb7f4c319fc
stack: frame={sp:0x7ffdc8e5e530, fp:0x0} stack=[0x7ffdc8660000,0x7ffdc8e5eac0)

.... snip ...

goroutine 1 gp=0xc0000061c0 m=0 mp=0x105eb00 [running]:
runtime.systemstack_switch()
runtime/asm_amd64.s:474 +0x8 fp=0xc000184750 sp=0xc000184740 pc=0x476508
runtime.main()
runtime/proc.go:171 +0x67 fp=0xc0001847e0 sp=0xc000184750 pc=0x445a67
runtime.goexit({})
runtime/asm_amd64.s:1695 +0x1 fp=0xc0001847e8 sp=0xc0001847e0 pc=0x478521

... snip ...

```
…

## What were you expecting to happen ?
to work

## Additional comments.
This is a Centos 7 with `6.8.2-1.el7.elrepo.x86_64`, selinux disabled