Avoid egrep in Salt code base

Summary:
On recent Fedora/RHEL releases, egrep raises a deprecation warning.
That warning fails selinux.fcontext_policy_present invocations.

This is especially problematic for the SELinux module for paas-docker role.
Don't record SELinux context created several times issues with nginx logs.

Patch is not needed on FreeBSD servers as on that userland, egrep works fine.

Ref T1991

Test Plan:

• Apply on WindRiver, check it's ignored
• Apply on Dwellers, run nginx config state

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1991

Differential Revision: https://devcentral.nasqueron.org/D3392