Simplify ssh config management

[ilia-db]

Changes

Simplify sshd config management, so we can easily re-use it. The main change is that we add one "include" directive to the system ssh config, and all new host configs are encapsulated in single files, which are easy to replace or add.

Here we also solve a separate logic of proper escaping env vars, as before we were letting new lines sneak into SetEnv directive, which is not allowed

Based on #4452

Why

Tests

[anton-107]

Is the scope != nil check redundant when err == nil?

[ilia-db]

The scope can be a default value with empty secrets

[anton-107]

we need to escape the backslashes too, and it should before escaping the quotes. consider the following test cases:

		{
			name:     "quote mid string",
			input:    `foo"bar`,
			expected: `foo\"bar`,
		},
		{
			name:     "quote end of string",
			input:    `foo bar"`,
			expected: `foo bar\"`,
		},
		{
			name:     "backslash end of string",
			input:    `foo bar\`,
			expected: `foo bar\\`,
		},
		{
			name:     "backslash before quote",
			input:    `foo\"bar`,
			expected: `foo\\\"bar`,
		},

[anton-107]

please double check the escaping sequence for SetEnv