Check that vm has no snapshots before changing service offering

[andrewbents]

closes #1492

[bwsw]

@wowshakhov Implemented as agreed. Please review.

[coveralls]

Pull Request Test Coverage Report for Build 1912

• 30 of 31 (96.77%) changed or added relevant lines in 6 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 53.813%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/app/vm/container/service-offering-dialog.container.ts	0	1	0.0%
<!--	Total:	30	31	96.77%	-->

Files with Coverage Reduction	New Missed Lines	%
src/app/shared/components/disk-offering/disk-offering-dialog/disk-offering-dialog.component.ts	2	78.85%
<!--	Total:	2		-->

Totals
Change from base Build 1904:	0.04%
Covered Lines:	9814
Relevant Lines:	16207

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💛 - Coveralls